fix: Define a default value for root ssh pub key

to avoid error when a value is not provided for root_ssh_key

openstack-ood/variables.pkr.hcl

 variable "root_ssh_key" {
   type        = string
+  default     = ""
   description = "The root key to use for ssh"
 }
 
@@ -87,4 +88,4 @@ variable "volume_size" {
   type        = number
   default     = 20
   description = "The default volume size for building iamge"
-}
\ No newline at end of file
+}