cli/snapshot : Add confirmation dialog to snapshot restore operation

When restoring a volume, the user is not prompted for confirmation.
Since restoring a volume rolls back the data to a previous point in time,
there is the potential for updates to be lost.
Hence it is better to display a confirmation dialogue during snapshot
restore operation.

Change-Id: I7b23eaeb43ad2aafa508e2ca5750d9b0fc7d6e36
BUG: 1133426
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/8525
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijaikumar Mallikarjuna <vmallika@redhat.com>
Reviewed-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>