GlusterFS: Delete active snapshot file on volume delete.

If a snapshot is taken of a volume that is attached to an active
instance, the volume file used by the instance will be switched to
the new snapshot file that is created.  When you delete the
snapshot, the base volume file will be merged with the snapshot
file and the base volume is deleted.  Upon a deleting the active
volume, the active snapshot file is not deleted because it does not
have the expected name that cinder is looking for, i.e.
volume-<uuid>.  Instead, the snapshot file has the name
volume-<uuid>.<snapshot-uuid>.  This patch looks at the volume info
file to find any active snapshot file and properly delete it when
the volume is deleted.

Change-Id: Ib0af4401d839ec3bd1eb3a81e1671811e0d4a288
Closes-Bug: #1300303
Merge "GlusterFS: Delete active snapshot file on volume delete."