glusterd: Use volume op-versions during volgen

Instead of using the cluster op-version, volume op-version is used to
enable open-behind during volgen. For doing this, the volume op-versions
are updated before regenerating the volfiles.

Change-Id: I675bb549bf7c7c0279030dca698fb530781addc6
BUG: 990830
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/5385
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>