Adding Incremental Backups

* Adding a config flag to register incremental runners
* Adding InnoBackupExIncremental runner and restore strategy.
* Adding save_metadata and load_metadata methods for
  swift storage strategy.
* Adding incremental backup/restore integration test.

Implements: blueprint incremental-backups

Change-Id: I1f0d4e5967097498f86a5052c33c55471e24a137
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
Merge "Adding Incremental Backups"
Merge "Initial support for single instance Cassandra Database"
Improve users/schemas extension exception handling

Reasons:
- for datastores which doesn't support
(or hadn't been implemented yet) extensions
like users/schemas. Trove-api service should perform
more restrict exception handling
- stacktraces are shown for all not supported operation.

Changes:
- imporved exception handling.