When creating external network, it belongs to no tenant.
Because of that, flow programming should be made in a way that requires
no segId.

routingProvider.programDefaultRouteEntry(node, dpid,
providerSegmentationId, defaultGatewayMacAddress, inetAddress,
actionForNodeDefaultRoute);

–

neutron net-create ext-net – --router:external=True
neutron subnet-create ext-net --allocation-pool start=172.16.18.200,end=172.16.18.210 \
--gateway=172.16.18.2 --enable_dhcp=False 172.16.18.0/24
neutron router-gateway-set demorouter ext-net