fix: add back variables removed in last release

Unfortunately our earlier `*-cluster` Terraform modules depened on the
`master` version of this module. Removing these variables in the 2.0.0
release broke those clients, so we're adding them back here without
using them.

Author: dvoros-castai

variables.tf

@@ -66,3 +66,15 @@ variable "skip_helm" {
   type        = bool
   default     = false
 }
+
+variable "cluster_region" {
+  description = "Not used. This variable is kept for backwards compatibility, will be removed in the future."
+  type        = string
+  default     = ""
+}
+
+variable "cluster_zone" {
+  description = "Not used. This variable is kept for backwards compatibility, will be removed in the future."
+  type        = string
+  default     = ""
+}