Add option for boot volume type for k8s node (#8256)

contrib/terraform/openstack/variables.tf

@@ -78,6 +78,10 @@ variable "master_volume_type" {
   default = "Default"
 }
 
+variable "node_volume_type" {
+  default = "Default"
+}
+
 variable "public_key_path" {
   description = "The path of the ssh pub key"
   default     = "~/.ssh/id_rsa.pub"