Correct use of deprecated argument to TF/OpenStack module

https://www.terraform.io/docs/providers/openstack/r/networking_router_v2.html#external_gateway

contrib/terraform/openstack/modules/network/main.tf

 resource "openstack_networking_router_v2" "k8s" {
   name                = "${var.cluster_name}-router"
   admin_state_up      = "true"
-  external_gateway = "${var.external_net}"
+  external_network_id = "${var.external_net}"
 }
 
 resource "openstack_networking_network_v2" "k8s" {