feature/snapshot : Interface to delete all snapshots belonging
to a system as-well-as to a particular volume.

Problem :
With the current design we can only delete a single snapshot.
And the deletion of volume which contains snapshot is not allowed.
Because of that user might be forced to delete all the snapshots
manually before he is allowed to delete a volume.

Solution:
Following is the interface with which user can delete
all the snapshots of a system or belonging to a particular volume.

        Syntax : gluster snapshot delete all

        *To delete all the snapshots present in a system

        Syntax : gluster snapshot delete volume <volname>

        *To deletes all the snapshot present in a volume specified.

========================================================================
Sample Output:

Case 1 : Deleting a single snapshot.
[root@snapshot-24 glusterfs]# gluster snapshot delete snap1
Deleting snap will erase all the information about the snap. Do you still want to continue? (y/n) y
snapshot delete: snap1: snap removed successfully

-----------------------------------------------------------------
Case 2 : Deleting all the snapshots in a Volume.
[root@snapshot-24 glusterfs]# gluster snapshot delete volume vol1
Volume (vol1) contains 9 snapshot(s).
Do you still want to continue and delete them?  (y/n) y
snapshot delete: snap2: snap removed successfully
snapshot delete: snap3: snap removed successfully
snapshot delete: snap4: snap removed successfully
snapshot delete: snap5: snap removed successfully
.
.
.

-----------------------------------------------------------------
Case 3 : Deleting all the snapshots in a system.
[root@snapshot-24 glusterfs]# gluster snapshot delete all
System contains 4 snapshot(s).
Do you still want to continue and delete them?  (y/n) y
snapshot delete: snap7: snap removed successfully
snapshot delete: snap8: snap removed successfully
snapshot delete: snap9: snap removed successfully
snapshot delete: snap10: snap removed successfully
========================================================================

Change-Id: Ifec8e128ab2011cbbba208376b9c92cfbe7d8d71
BUG: 1112613
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/8162
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>