glusterd : enable/disable quota using volume set cmd deprecated.

Using the command 'gluster volume set <volname> quota {on|off}'
doesn't really enable/disable quota. So, a message to the user is
provided. Message asks the user to use 'gluster volume
quota <volname> {enable|disable}' instead.

Improvement of the previous method to solve the issue.

Change-Id: Ibeda35bb49ec7b541d53e592b48fd132380fce76
BUG: 1024626
Signed-off-by: Anuradha Talur <atalur@redhat.com>
Reviewed-on: http://review.gluster.org/6512
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>