Comment 8 for bug 1399684

Just as a quick aside the variable is generated from data found in the provider_networks array set in the openstack_user_config.yml it is possible however to override this default by setting the following in user_variables.yml

neutron_ml2_conf_ini_overrides:
  ml2:
    tenant_network_types: <SOME-NETWORK-TYPE>

In this way you can control the exact type of networks available via neutron.