Reset parameter group values when they are removed from config

[ryndaniels]

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Closes #661

Release note for CHANGELOG:

BUG FIXES:
* resource/aws_db_parameter_group: Resets parameters to default values when they are removed from Terraform config

Output from acceptance testing:

• New test - before the fix:

$ make testacc TESTARGS="-run=TestAccAWSDBParameterGroup_resetParameters"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./... -v -parallel 20 -run=TestAccAWSDBParameterGroup_resetParameters -timeout 120m
?       github.com/terraform-providers/terraform-provider-aws   [no test files]
=== RUN   TestAccAWSDBParameterGroup_resetParameters
=== PAUSE TestAccAWSDBParameterGroup_resetParameters
=== CONT  TestAccAWSDBParameterGroup_resetParameters
--- FAIL: TestAccAWSDBParameterGroup_resetParameters (66.42s)
    testing.go:569: Step 1 error: After applying this step and refreshing, the plan was not empty:
        
        DIFF:
        
        UPDATE: aws_db_parameter_group.bar
          arn:                      "arn:aws:rds:us-west-2:187416307283:pg:parameter-group-test-terraform-5010972312428257489" => "arn:aws:rds:us-west-2:187416307283:pg:parameter-group-test-terraform-5010972312428257489"
          description:              "Managed by Terraform" => "Managed by Terraform"
          family:                   "mysql5.6" => "mysql5.6"
          id:                       "parameter-group-test-terraform-5010972312428257489" => "parameter-group-test-terraform-5010972312428257489"
          name:                     "parameter-group-test-terraform-5010972312428257489" => "parameter-group-test-terraform-5010972312428257489"
          parameter.#:              "3" => "1"
          parameter.0.apply_method: "immediate" => "immediate"
          parameter.0.name:         "character_set_client" => "character_set_server"
          parameter.0.value:        "utf8" => "utf8"
          parameter.1.apply_method: "immediate" => ""
          parameter.1.name:         "character_set_results" => ""
          parameter.1.value:        "utf8" => ""
          parameter.2.apply_method: "immediate" => ""
          parameter.2.name:         "character_set_server" => ""
          parameter.2.value:        "utf8" => ""
        
        
        
        STATE:
        
        aws_db_parameter_group.bar:
          ID = parameter-group-test-terraform-5010972312428257489
          provider = provider.aws
          arn = arn:aws:rds:us-west-2:187416307283:pg:parameter-group-test-terraform-5010972312428257489
          description = Managed by Terraform
          family = mysql5.6
          name = parameter-group-test-terraform-5010972312428257489
          parameter.# = 3
          parameter.0.apply_method = immediate
          parameter.0.name = character_set_client
          parameter.0.value = utf8
          parameter.1.apply_method = immediate
          parameter.1.name = character_set_results
          parameter.1.value = utf8
          parameter.2.apply_method = immediate
          parameter.2.name = character_set_server
          parameter.2.value = utf8
FAIL
FAIL    github.com/terraform-providers/terraform-provider-aws/aws       67.852s
make: *** [testacc] Error 1

• New test - after the fix:

$ make testacc TESTARGS="-run=TestAccAWSDBParameterGroup_resetParameters"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./... -v -parallel 20 -run=TestAccAWSDBParameterGroup_resetParameters -timeout 120m
?       github.com/terraform-providers/terraform-provider-aws   [no test files]
=== RUN   TestAccAWSDBParameterGroup_resetParameters
=== PAUSE TestAccAWSDBParameterGroup_resetParameters
=== CONT  TestAccAWSDBParameterGroup_resetParameters
--- PASS: TestAccAWSDBParameterGroup_resetParameters (66.51s)
PASS
ok      github.com/terraform-providers/terraform-provider-aws/aws       67.490s

• All tests - after the fix:

$ make testacc TESTARGS="-run=TestAccAWSDBParameterGroup_"         ==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./... -v -parallel 20 -run=TestAccAWSDBParameterGroup_ -timeout 120m
?       github.com/terraform-providers/terraform-provider-aws   [no test files]
=== RUN   TestAccAWSDBParameterGroup_importBasic
=== PAUSE TestAccAWSDBParameterGroup_importBasic
=== RUN   TestAccAWSDBParameterGroup_limit
=== PAUSE TestAccAWSDBParameterGroup_limit
=== RUN   TestAccAWSDBParameterGroup_basic
=== PAUSE TestAccAWSDBParameterGroup_basic
=== RUN   TestAccAWSDBParameterGroup_resetParameters
=== PAUSE TestAccAWSDBParameterGroup_resetParameters
=== RUN   TestAccAWSDBParameterGroup_Disappears
=== PAUSE TestAccAWSDBParameterGroup_Disappears
=== RUN   TestAccAWSDBParameterGroup_namePrefix
=== PAUSE TestAccAWSDBParameterGroup_namePrefix
=== RUN   TestAccAWSDBParameterGroup_generatedName
=== PAUSE TestAccAWSDBParameterGroup_generatedName
=== RUN   TestAccAWSDBParameterGroup_withApplyMethod
=== PAUSE TestAccAWSDBParameterGroup_withApplyMethod
=== RUN   TestAccAWSDBParameterGroup_Only
=== PAUSE TestAccAWSDBParameterGroup_Only
=== RUN   TestAccAWSDBParameterGroup_MatchDefault
=== PAUSE TestAccAWSDBParameterGroup_MatchDefault
=== CONT  TestAccAWSDBParameterGroup_importBasic
=== CONT  TestAccAWSDBParameterGroup_MatchDefault
=== CONT  TestAccAWSDBParameterGroup_Disappears
=== CONT  TestAccAWSDBParameterGroup_resetParameters
=== CONT  TestAccAWSDBParameterGroup_namePrefix
=== CONT  TestAccAWSDBParameterGroup_basic
=== CONT  TestAccAWSDBParameterGroup_limit
=== CONT  TestAccAWSDBParameterGroup_Only
=== CONT  TestAccAWSDBParameterGroup_generatedName
=== CONT  TestAccAWSDBParameterGroup_withApplyMethod
--- PASS: TestAccAWSDBParameterGroup_Disappears (26.55s)
--- PASS: TestAccAWSDBParameterGroup_Only (29.57s)
--- PASS: TestAccAWSDBParameterGroup_MatchDefault (36.03s)
--- PASS: TestAccAWSDBParameterGroup_generatedName (37.48s)
--- PASS: TestAccAWSDBParameterGroup_withApplyMethod (39.59s)
--- PASS: TestAccAWSDBParameterGroup_namePrefix (39.80s)
--- PASS: TestAccAWSDBParameterGroup_importBasic (41.77s)
--- PASS: TestAccAWSDBParameterGroup_resetParameters (67.72s)
--- PASS: TestAccAWSDBParameterGroup_basic (70.27s)
--- PASS: TestAccAWSDBParameterGroup_limit (73.93s)
PASS
ok      github.com/terraform-providers/terraform-provider-aws/aws       75.404s

[ryndaniels]

Closing as a dupe of #5728! 👍

[ghost]

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