glusterd/snapshot: Restore cleanup

If restores fails for some reason then we should revert
the restore operation. To do so we take the backup of
vols folder before doing a restore and if the restore
fails then we revert the changes done.

Change-Id: I97f72aec3a34fc122bf137beb336e94db3a04dff
BUG: 1061685
Signed-off-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-on: http://review.gluster.org/7548
Reviewed-by: Santosh Pradhan <spradhan@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>