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
Merge "Adding Incremental Backups"