Try fix issue where ports doesn't get an ip assigned (#9345)

Co-authored-by: Jonathan Süssemilch Poulain <jonathan@sofiero.net>

contrib/terraform/openstack/modules/compute/variables.tf

@@ -189,3 +189,7 @@ variable "port_security_enabled" {
 variable "force_null_port_security" {
   type = bool
 }
+
+variable "private_subnet_id" {
+  type = string
+}