google-cloud: Set disk.device_name to match API default

* Terraform provider "google" plugin releases leave the disk
device_name as "" by default. Recently the API has started to
set a default name "persistent-disk-0". Plan and apply show
all instance groups need to be recreated to "fix" the name
* Impact: Controller and worker instance groups are deleted
and recreated, deleting data on controllers and bringing
down clusters
* Fix: Explicitly set the disk_name to persistent-disk-0 so
that terraform finds no diff needs to be applied.
* #34
* hashicorp/terraform-provider-google#574

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

@@ -55,6 +55,9 @@ resource "google_compute_instance_template" "controller" {
     boot         = true
     source_image = "${var.os_image}"
     disk_size_gb = "${var.disk_size}"
+    // Set explicit name to match the new default name set by the API.
+    // https://github.com/terraform-providers/terraform-provider-google/issues/574
+    device_name = "persistent-disk-0"
   }
 
   network_interface {

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

@@ -55,6 +55,9 @@ resource "google_compute_instance_template" "worker" {
     boot         = true
     source_image = "${var.os_image}"
     disk_size_gb = "${var.disk_size}"
+    // Set explicit name to match the new default name set by the API.
+    // https://github.com/terraform-providers/terraform-provider-google/issues/574
+    device_name = "persistent-disk-0"
   }
 
   network_interface {