GlusterFS: Handle deletion of snapshot with no backing file

If a glusterfs volume is in-use, nova is called to delete a
volume snapshot.  It is possible for a timeout to occur, since
cinder is polling for nova task progress.  In such case, a
GlusterfsException is thrown, but nova continues to process the
snapshot delete and commits the changes.  Cinder is not aware of
this, and the actual snapshot entry in cinder is prevented from
being deleted, since no backing file exists for the out-of-sync
snapshot.  This patch allows a snapshot with no backing file to
be deleted from cinder and updates the volume info file.

Change-Id: I83c8a7242199edbfd1897297589ac7deb42c5865
Closes-Bug: #1311182
Merge "GlusterFS: Handle deletion of snapshot with no backing file"