glusterd: Allow bumping down a peer's op-version during probe

Earlier, a peer running a higher op-version couldn't be probed into a
cluster running at a lower op-version. This created issues when trying
to expand an upgraded cluster. This patch changes this behaviour.

The cluster no longer rejects a peer being probed if its op-version is
higher than the cluster op-version. The peer will reduce its op-version
if it doesn't have any volumes. If the peer contains volumes and needs
to reduce its op-version, it fails the handshake and the probe fails.

Change-Id: I12c6c873922799e1557b7184e956baea643d0dea
BUG: 1005038
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/5715
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>