Support Hortonworks' broken repository layout for HDP 2.1

This allows a user to:
- [x] Select HDP 2.0 and get updates to 2.0 releases, only
- [x] Select HDP 2.1 and get updates to latest
- [x] Select a specific HDP 2.0 release and get updates to only that version (pinning)
- [x] Select HDP "2" and get updates to latest (so 2.2, if it were released, today)