Fix implicit map assignments to be explicit

* Terraform v0.12 will require map assignments be explicit,
part of v0.12 readiness

google-cloud/container-linux/kubernetes/workers/workers.tf

@@ -28,7 +28,7 @@ resource "google_compute_instance_template" "worker" {
   machine_type     = "${var.machine_type}"
   min_cpu_platform = "Intel Haswell"
 
-  metadata {
+  metadata = {
     user-data = "${data.ct_config.worker-ignition.rendered}"
   }