Modify cluster template to allow setting network tags on default-pool

This adds the following tags to the default pool:
* the cluster_network_tag (gke-${var.name})
* the pool-specific network tag (gke-${var.name}-default-pool)
* user-specified tags from var.node_pools_tags["all"]\
* user-specified tags from var.node_pools_tags["default-pool"]

autogen/main/cluster.tf.tmpl

@@ -288,6 +288,13 @@ resource "google_container_cluster" "primary" {
 
       service_account = lookup(var.node_pools[0], "service_account", local.service_account)
 
+      tags = concat(
+        lookup(local.node_pools_tags, "default_values", [true, true])[0] ? [local.cluster_network_tag] : [],
+        lookup(local.node_pools_tags, "default_values", [true, true])[1] ? ["${local.cluster_network_tag}-default-pool"] : [],
+        lookup(local.node_pools_tags, "all", []),
+        lookup(local.node_pools_tags, "default-pool", []),
+      )
+
       dynamic "workload_metadata_config" {
         for_each = local.cluster_node_metadata_config