GlusterFS: Fix create/restore backup

Allow create_backup for GlusterFS volumes, but
only when snapshots do not exist.  (For now.)

Restore is a no-op for the driver, so allow it.

Related-Bug: 1247743
Closes-Bug: 1275977

Change-Id: I50b8f6cac684c967c7374bb43247a396ce936157
Merge "GlusterFS: Fix create/restore backup"