Convert configuration elements to be all hypens, ensure all plugin definitions use underscores

This is a great idea to make the DX and documentation much more clear and easy to follow when we tell people about the format of the drupalci.yml file.

We'll need to do a few things:

1. convert all instances of underscores to hypens in configuration keys in the code and in tests.
2. Ensure that as we read in old drupalci.yml files that if we see a config key that doesnt match a plugin name, that we convert it's underscores into hyphens as well.
3. Six months from now when all the old builds are gone we can get rid of (2).