Do not default to deprecated VIF driver

LibvirtHybridOVSBridgeDriver has been deprecated in Nova since
Grizzly, and is being removed in Icehouse. The new default
(and somewhat universal) VIF driver for libvirt is
LibvirtGenericVIFDriver, so default to that instead.

Change-Id: I847a0de913c23fbef8a12f5258774268f605c87d
Merge "Do not default to deprecated VIF driver"