Refactor #765: Convert Pulp to use MongoEngine
convert RepoUnitAssociationManager.unassociate_by_criteria to use MongoEngine models when possible
in RepoUnitAssociationManager.unassociate_by_criteria Add a switch that checks if all of the types on the repo have been converted to models then call branch to a new call the uses the the MongoEngine repo model to perform the delete and then calls the importer with the repo_content_unit objects.
- a RepoUnitAssociationManager.unassociate_by_criteria that will use MongoEngine models if all the types in the given repo have been converted
- This can use pulp_docker for testing