terraform/gcp: Add ingress_whitelist (#8590)

Also, do not create unneeded resources (target pools are charged and should
only be created when needed).

contrib/terraform/gcp/variables.tf

@@ -90,3 +90,8 @@ variable api_server_whitelist {
 variable nodeport_whitelist {
   type = list(string)
 }
+
+variable "ingress_whitelist" {
+  type = list(string)
+  default = ["0.0.0.0/0"]
+}