glusterd/snapshot: Change file-system uuid to file-system label

Problem: In XFS changing file-system UUID with xfs_admin is causing
too much delay with large file-system. The time taken by xfs_admin
tool to change UUID is directly proportional to the size of the file
system.

Cause: In XFS file-system UUID is stored in file-system superblock.
Therefore for chaning UUID all the superblock needs to be changed.

Fix: Instead of using file-system UUID use file-system label.

Change-Id: Ifb4c668fb29cfc1c89d9b221abc8d09dc09589ec
BUG: 1115107
Signed-off-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-on: http://review.gluster.org/8215
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>