Bug # 1199853 - Add libvirt_vif_driver setting for OVS Bridge.

Added the libvirt_vif_driver setting for use with the OVS plug-in to the
nova.conf file provided in the guide. This setting is required for OVS
networking to function correctly, as the guide does not attempt to use
the linux bridge plug-in no alternative required.

Change-Id: I6f09068efcf11232c8a9ca064c56eeb207d11f7b
Fixes: 1199853
Merge "Bug # 1199853 - Add libvirt_vif_driver setting for OVS Bridge."