Concent VM: Move configuration env variables to config.yml

[cameel]

In #278 we added concent-vm/config.yml. At the time when that issue was defined the intention was to move all the configuration to that file. #332 added a lot of new variables and those were not handled in #278. They should be added to config.yml too.

• For each variable add an entry in config.yml. Make it lower case.
• Load values from config.yml in Vagrantfile.
• Remove support for the current variables.
• Update README.
• Provide a template file with defaults that can be used as a starting point by someone configuring the machine.

If processing YAML in Vagrantfile turns out to be complicated (e.g. if it requires installing extra Ruby packages) we can also discuss alternative solutions. E.g. keeping the variables and just putting them in a file to be sourced by the Vagrantfile.