Removes unecessary call to rbd.Image

Removed duplicate and unprotected call to rbd.Image
in rbd driver which can result in an uncaught exception
if the rbd image no longer exists.

Also added unit test for _delete_backup_snaps()

Closes-Bug: 1323203
Change-Id: I89eedb0d0276a985b4d496093df2eee2ce067ec1