terraform/gcp: Allow to use preemptible VM instances (#8480)

contrib/terraform/gcp/variables.tf

@@ -44,6 +44,11 @@ variable "master_sa_scopes" {
   default = ["https://www.googleapis.com/auth/cloud-platform"]
 }
 
+variable "master_preemptible" {
+  type    = bool
+  default = false
+}
+
 variable "worker_sa_email" {
   type    = string
   default = ""
@@ -54,6 +59,11 @@ variable "worker_sa_scopes" {
   default = ["https://www.googleapis.com/auth/cloud-platform"]
 }
 
+variable "worker_preemptible" {
+  type    = bool
+  default = false
+}
+
 variable ssh_pub_key {
   description = "Path to public SSH key file which is injected into the VMs."
   type        = string