module fails if `external_network_name` is not set

This should be an optional variable as you may not want a gateway or external network. In the example below I did not set the variable external network for the module as I did not need it.

Error: Your query returned more than one result. Please try a more specific search criteria

  on .terraform/modules/management_network/main.tf line 26, in data "openstack_networking_network_v2" "external_network":
  26: data "openstack_networking_network_v2" "external_network" {


Releasing state lock. This may take a few moments...
make: *** [Makefile:97: plan] Error 1

Setting the variable, though unused, allows the module to behave as expected