USS : Display only the activated snapshots

Instead of displaying all the snapshots in the uss world,
it is better if we display only the activated snapshots.

Change-Id: I70d3ec212b62ec15956ae3e826bc4201d8dedd17
BUG: 1155042
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/8958
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Krishnan Parthasarathi <kparthas@redhat.com>