Use subnet_dns_publish_fixed_ip Neutron extension

There are multiple Neutron extensions that can be used for DNS
integration. The "dns" extension has only minimum functionality, for
most deployments either "dns_domain_ports" or
"subnet_dns_publish_fixed_ip" would be used, with the latter being a
superset of the two others [0].

[0] https://docs.openstack.org/neutron/xena/admin/config-dns-int-ext-serv.html#use-case-3-ports-are-published-directly-in-the-external-dns-service



Signed-off-by: Dr. Jens Harbott <harbott@osism.tech>
Change-Id: I1056bddfd49265ffe21b4d000898d72bba3ebfad