fix(controller): import image from remote registry

This change allows us to import an image from a remote repository,
thanks to a new feature implemented in
https://github.com/deis/docker-registry/compare/repository-import.

Though the infrastructure in place allows us to import an image from the public index,
the endpoints are closed-source and therefore are not implmented in the upstream
repository-import branch.
fix(controller): add back REGISTRY_URL
fix(controller): clean up publish_release params

Publishing a release should only know about a source, a dictionary of
configuration, and a target.
refactor(controller): clean up Release.new
refactor(controller): remove urlparse logic

The logic has been moved over to the registry, so it's not needed any more.
fix(client): remove latest tag appending

Registry repository imports does not like tags.
style(controller): fix flake8
test(controller): mock import_repository task