Support for JUNO - Phase 1
JUNO release doesn't support libvirt_vif_driver configuration in nova.conf vif_driver is set to LibvirtGenericVIFDriver.
plug/unplug/get_config api from this class doesn't support opencontrail. Till opencontrail vif_driver is upstreamed, overwrite the vif_driver with VRouterVIFDriver.

This code will be removed after OpenContrail vif driver is upstreamed

Change-Id: I84b2c85d757995504e66817d87458febb231a437