[WIP] Move ConfigurationScriptSource#authentication to GitRepository

[Fryguy]

This is an attempt to deal with the "double ownership problem" with ConfigurationScriptSource#authentication and GitRepository. The idea is that in the model we can delegate authentication stuff to the GitRepository. Thus, this migration first ensures that a git_repository exists, then moves the authentication over to it.

In GitRepository model, while it might have it's own authentication via AuthenticationMixin, we also add a separate belongs_to :authentication which is a referenced authentication. This way we can support both directly owned authentications and not-owned authentications.

cc @gtanzillo @carbonin @agrare @NickLaMuro

[Fryguy]

Missing specs, but wanted to just get it out there.

[miq-bot]

Checked commit Fryguy@a09a688 with ruby 2.4.6, rubocop 0.69.0, haml-lint 0.20.0, and yamllint 1.10.0
2 files checked, 4 offenses detected

db/migrate/20190723023241_move_configuration_script_source_authentication_to_git_repository.rb

• ❗ - Line 13, Col 33 - Rails/ReflectionClassName - Use a string value for class_name.
• ❗ - Line 14, Col 33 - Rails/ReflectionClassName - Use a string value for class_name.
• ❗ - Line 26, Col 49 - Layout/ExtraSpacing - Unnecessary spacing detected.
• ❗ - Line 7, Col 33 - Rails/ReflectionClassName - Use a string value for class_name.

[carbonin]

Won't we lose the SCM credentials for existing ConfigurationScriptSources with this change? Should this also create GitRepository instances in the case that we don't have one currently? I don't see the authentication getting moved over in the create step in ManageIQ/manageiq#19027.