Conductor proxies host db access for guests

Previously, instances updated their status by
updating the database on the host directly.
Necessarily, each instance would need access to the
database to stay updated.

Trove's new conductor service eliminates that need by
working as a proxy for those instances. By sending a heartbeat
to conductor via RPC, conductor updates the database
on the host on behalf of the instance.

As backups also made use of the host database, the backup
code has been refactored to take richer inputs to remove
the need to query the host database, and now conductor is
also used to submit updates to backup states.

Implements: blueprint trove-conductor

Change-Id: I4cb34baedd0e3a50051f9e66de95c9028c66e4b5