glusterd/snapshot: mount snapshot volume with read-only option

Snapshot volumes are readonly. If you mount the volume to the client it
doesn't allow writes, but its attributes are rw which contradicts the
functionality.

mount script should set read-only attributes for snapshot volumes.

Change-Id: I056253abd8dfe7b2b43a064fbdbd9c16b8eca679
BUG: 1132946
Signed-off-by: vmallika <vmallika@redhat.com>
Reviewed-on: http://review.gluster.org/8518
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Krishnan Parthasarathi <kparthas@redhat.com>