Set default value for "data_volume" variable

vars.tf

@@ -25,7 +25,7 @@ variable "external_network" {default = "dmznet"}
 variable "ohpc_user" {default = "centos"}
 variable "ood_user" {default = "centos"}
 variable "ssh_private_key" {default = "~/.ssh/id_rsa"}
-variable "data_volume" {default = ""}
+variable "data_volume" {default = "xdmod_queue_data_volume"}
 
 # variables for node creation module
 variable "image_compute" {default = "compute-v1"}