set LibvirtHybridOVSBridgeDriver for neutron

Initial testing of a havana deployment indicates that
libvirt_vif_driver has to be set to
 LibvirtHybridOVSBridgeDriver for neutron with ovs
 on neutron to work.
Merge pull request #149 from bodepd/update_vif_driver

set LibvirtHybridOVSBridgeDriver for neutron