GlusterFS: Delete volume-<uuid>.info file when volume is deleted.

glusterfs creates a file named volume-<uuid>.info to hold volume
data, mapping cinder snapshot UUIDs and filenames. When the
volume and its associated snapshots are both deleted, this file
remains. This patch deletes the volume-<uuid>.info (if one exists)
when the volume is deleted.

Change-Id: Iefc8bd4768b30a75c648c4689d935daeab3afe98
Closes-Bug: #1299182
Merge "GlusterFS: Delete volume-<uuid>.info file when volume is deleted."