Fixes Neutron configuration

Fixes configuration for interface_driver. The driver configured was
nova.networklinux_net.LinuxOVSInterfaceDriver which is for flat
networks. In our labs cluster we are supporting L3 layer
functionality which is provided by
neutron.agent.linux.interface.OVSInterfaceDriver.

Change-Id: I71d971a27afea377e27eb8acbb46d283a62b5aa3