Support setting additional labels on slave containers

[francoisferrand]

This patch adds an extraLabels fields to DockerTemplateBase, which is used to specify additional docker labels to set on slave containers.

This can be used for example to logically group containers, or to setup scheduling constraints on swarm.

See also: https://issues.jenkins-ci.org/browse/JENKINS-45227

[pjdarton]

Two main issues:

1. Naming
   Choosing names for things is never as straightforward as it could be and, sadly, this is no exception 😬
   Slave nodes (and templates that create them) already have a field for "labels", so having a field called "extraLabels" sounds like it's going to add to the labels field rather than the docker container labels.
   I think we need a big rename here and call the new field "extraDockerLabels" rather than just "extraLabels" (plus extra text in the html help file).
   Hopefully a big search & replace turning xtraLabels to xtraDockerLabels will do 95% of the work...

2. Configuration round trip
   The getter/setter pairing turns nulls to empties and hashCode/equals treats this as a change, resulting in a unit-test failure: https://ci.jenkins.io/job/Plugins/job/docker-plugin/view/change-requests/job/PR-747/1/testReport/
   Either persist nulls as nulls and empties as empties ... or ensure that empties are always stored as nulls.
   Personally, I quite like the idea of absent/unused configuration fields simply not appearing at all in the saved config (as it makes for smaller more efficient files), which means we need nulls when we're turned into XML.

[pjdarton]

Thanks for the PR - enhancements such as this are always welcome 😀

I've done a review - please resolve the changes requested and ping me once you think it's there.

Hint: If the automated build on ci.jenkins.io fails for reasons that you don't think are your fault, you can make it retry the build by closing the PR, waiting 10 minutes, and then re-opening the PR again (you don't need to create a new PR). You can also do a local build & test by doing mvn install on a linux machine that's got Java 8, [a recent] maven and [a recent] docker installed.

[pjdarton]

Check the doCheckExtraDockerLabelsString method.

[francoisferrand]

All changes done, can you please review?