Return subnet_id as defined in kubespray.tf

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

@@ -2,6 +2,6 @@ output "router_id" {
   value = "${openstack_networking_router_interface_v2.k8s.id}"
 }
 
-output "network_id" {
+output "subnet_id" {
   value = "${openstack_networking_subnet_v2.k8s.id}"
 }