Revert "remove unsupported libvirt_vif_driver nova.conf setting"

This reverts commit 77ebfa6364ab48c2f448c6a9495577622f569ec3.

LibvirtHybridOVSBridgeDriver is required for functional security groups.

Fixes: rhbz#1052124
Change-Id: I1f4b9bb653666b1f6e6d6496fd3041cbd74568eb
Merge "Revert "remove unsupported libvirt_vif_driver nova.conf setting"" into havana