Switch job configuration section to use a checkbox

This pull request changes the plugins job configuration from using a _section_ to an _optionalBlock_. In effect, it makes the plugins configuration fall inline with the convention used by most other plugins.
I think I could probably get rid of the need for _configEnabled_ altogether.
Which means I should be able to remove the method, the new field and the new constructor argument.

But it's still possible it will break the plugins you mentioned as it would still require _configEnabled_ to be included in the _StaplerRequest_ object being passed into the _newInstance_ method. This is because when the job config is saved, we need to have at least one parameter to know if the checkbox is ticked or not.
I have almost no knowledge of how these plugins work though, so I could be very wrong here.


Regarding the tests, I have updated the existing ones and added a new one to test the new constructor parameter.