Add support for config diff and versioning

Some sort of config diff command support would help a great deal to know the set of changes being made before doing a deploy operation. Similarly it would be very useful to be able to revert to previous config versions. To support both of these one approach would be to lay down a git repository under the kolla config space. This would allow us to do all sort of nice version control / diff operations that git supports, and we could also then easily support pushing / pulling with remote repos, etc.