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
Initial support for single instance Cassandra Database

Reasons
- This code adds ability to create/delete instance
with mongodb service type. No other operations supported.
- Trove should support multiple extensions. One of the is NoSQL support.
Main goal is to add support for Apache Cassandra NoSQL database.

Changes
This code gives ability to create/delete instance
with cassandra datastire type. No other operations supported.

Change-Id: Ib48f6392765af49c2df1447c3df46675825d210f
Implements: blueprint cassandra-db-support