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: 1145083
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>
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/8798
Reviewed-by: Vijaikumar Mallikarjuna <vmallika@redhat.com>
doc : man page and admin-guide for newly introduced snapshot delete option

Change-Id: Iab5e7f63d673a2040e8d83ab7280121ff468836e
BUG: 1145084
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/8378
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/8799
Reviewed-by: Vijaikumar Mallikarjuna <vmallika@redhat.com>