glusterd-volgen: Enable open-behind based on op-version

Backport of patch on master branch, under review at
        http://review.gluster.org/4866

This patch enables the open-behind by default only when the op-version
allows it. Also the volume op-version calculations take account of this
enablement.

BUG: 954256
Change-Id: Ie739bc23ba90ec2f009feecef28187912a37487c
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/5095
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
glusterd-volgen: Improve volume op-versions calculation

Backport of patch on master branch, under review at
        http://review.gluster.org/4952

Volume op-versions calculations now take into account if an option,
 a. enables/disables an xlator, or
 b. is a boolean option.

This prevents op-versions from being updated when a feature is disabled.

BUG: 954256
Change-Id: Ic68032b9e55a3f0191f8fc3ecd6b5ced385ad943
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/5094
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>