Remove the cluster_domain_suffix variable

* Drop support for `cluster_domain_suffix` customization and
always use `cluster.local`. Many components in the Kubernetes
ecosystem assume this default suffix and its very rare to be
setting a special value here these days
* Cleanup a few variables that are seldom used

google-cloud/flatcar-linux/kubernetes/workers/butane/worker.yaml

@@ -98,7 +98,7 @@ storage:
           cgroupDriver: systemd
           clusterDNS:
             - ${cluster_dns_service_ip}
-          clusterDomain: ${cluster_domain_suffix}
+          clusterDomain: cluster.local
           healthzPort: 0
           rotateCertificates: true
           shutdownGracePeriod: 45s

google-cloud/flatcar-linux/kubernetes/workers/variables.tf

@@ -96,13 +96,7 @@ variable "node_taints" {
   default     = []
 }
 
-# unofficial, undocumented, unsupported, temporary
-
-variable "cluster_domain_suffix" {
-  type        = string
-  description = "Queries for domains with the suffix will be answered by coredns. Default is cluster.local (e.g. foo.default.svc.cluster.local) "
-  default     = "cluster.local"
-}
+# advanced
 
 variable "accelerator_type" {
   type        = string

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

@@ -111,7 +111,6 @@ data "ct_config" "worker" {
     kubeconfig             = indent(10, var.kubeconfig)
     ssh_authorized_key     = var.ssh_authorized_key
     cluster_dns_service_ip = cidrhost(var.service_cidr, 10)
-    cluster_domain_suffix  = var.cluster_domain_suffix
     node_labels            = join(",", var.node_labels)
     node_taints            = join(",", var.node_taints)
   })