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>
> Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>

Change-Id: Ida7340ea18a558cd15f25f2787a9794e287b17bd
BUG: 1175694
Reviewed-on: http://review.gluster.org/9296
Reviewed-by: Vijaikumar Mallikarjuna <vmallika@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>
Tested-by: Raghavendra Bhat <raghavendra@redhat.com>