The vif_driver option was deprecated in Ocata:
I599f3449f18d2821403961fb9d52e9a14dd3366b
And can now be removed. The only supported networking
backend is neutron + ovs.
Related to blueprint remove-nova-network
Co-Authored-By: Naichuan Sun <naichuan.sun@citrix.com>
Change-Id: Ia977f115335f00bc36249fa67437b4336d524251