cli: Add a cli command to enable/disable barrier

This patch adds a new
 'gluster volume barrier <VOLNAME> {enable|disable}'
cli command. This helps in testing the brick op code path when testing
the barrier xlator.
This patch can be reverted later if not required for end users.
Change-Id: Icd86a2d13e7f276dda1ecbb2593d60638ece7dcd
BUG: 1060002
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/6958
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>