Add datastore version to backups

This fix adds datastore information to the backup APIs.
These API includes backup list, backup list by instance and
backup show.

Also this fix enhances backup-list to optionally receive a
datastore to filter the backups by.

The integration tests will fail until the update for the client
gets merged: https://review.openstack.org/#/c/90462/

Change-Id: Icfec975b92cd9523e639ad6a2d6787ee4d4cb39d
Implements: blueprint backup-metadata