RPM Support

Since DistributionTree is tied to a Repository, we need to create a new DistributionTree for each. In order to achieve that, we should introduce a new field repository_id and have it as a unique index.

Also fix the Addon and Variant models to point to RpmRepository rather than the master model.

Migration is needed, during which a new DistributionTree will be created in case it belongs to multiple repos.

Make sure that only one DistributionTree can be in a repo by performing a check in finalize_repo_version. The newest (incoming) DistributionTree always wins.

All the current problems with Distribution Trees are due to the fact that they have a circular dependency from a Variant to a main repo a Distribution Tree belongs to. The main tree needs to be managed by user like a normal RPM repo, so Distribution Tree Varian and the main repo should be detached. It also solves problems with copy of a DistributionTree and a repo deletion.