Describe the issue: 

From engineering RFE: https://bugzilla.redhat.com/show_bug.cgi?id=1600961

It would be great to be able to create a Git backed automation domain through the API. An example use case is bootstraping a ManageIQ appliance with domains managed with Git. This would also be helpful in a CI/CD approach as only the dev environment would have non Git domains.

The Git repository should not be exposed on its own. I would expect an API call similar to:

POST /api/automate_domains
{
  "name": "AwesomeDomain",
  "description": "Bob's awesome domain",
  "source": "git",
  "git_repository": {
    "url": "https://github.com/Bob/AwesomeDomain",
    "branch": "production"
  }
}

The Git repository object would be created by the backend because the user creates a Git backed domain. Then, in a CI/CD context, there would be developers' domains, with a merge policy. Then, a tool like Jenkins would call the refresh_from_source action on the repo when the content of the repo changes.