add master_volume_type variable (#6524)

contrib/terraform/openstack/variables.tf

@@ -74,6 +74,10 @@ variable "gfs_volume_size_in_gb" {
   default = 75
 }
 
+variable "master_volume_type" {
+  default = "Default"
+}
+
 variable "public_key_path" {
   description = "The path of the ssh pub key"
   default     = "~/.ssh/id_rsa.pub"