As described in [0] a new attribute ``dns_publish_fixed_ip`` is added
to subnets, allowing to specify directly whether DNS records should be
published for this subnet. This overrides the previous behaviour that
makes this decision based on various properties of the network that
the subnet is contained in, see [1].
[0] https://launchpad.net/bugs/1784879
[1] https://docs.openstack.org/neutron/latest/admin/config-dns-int-ext-serv.html
Change-Id: I14605ead2694d9e9422b3d7b519aed2e3c340e2a
Partial-Bug: 1784879