Fixes for GKE

* Fix non-empty plan when spot instances are used
* Add cluster_id and recommend its use, as this prevents inconsitencies
  when only cluster is recreated (with no changes on node pool)

modules/gke-nodepool/variables.tf

@@ -14,6 +14,12 @@
  * limitations under the License.
  */
 
+variable "cluster_id" {
+  description = "Cluster id. Optional, but providing cluster_id is recommended to prevent cluster misconfiguration in some of the edge cases."
+  type        = string
+  default     = null
+}
+
 variable "cluster_name" {
   description = "Cluster name."
   type        = string