Comment 1 for bug 1843104

It looks like this was caused by https://review.opendev.org/#/c/655208/, which changes the handling of defaults for config in horizon. Since we override the OPENSTACK_NEUTRON_NETWORK variable in local_settings.py, but do not include the default for 'default_dns_nameservers'. A simple workaround for kolla-ansible is to include the default empty list value in our override.

I think this is a bug in horizon though - they should still tolerate missing defaults in dict format config variables.