Comment 0 for bug 1834306

Add a new option ``dns_nameservers`` in the subnet sections for the undercloud ctlplane network. By default the option is not set, and in that case the ``undercloud_nameservers`` are used, maintaining the current behaviour.

This decouples the nameservers used on the undercloud and the ones used for the overcloud nodes. In a DCN setting different DNS server may be preferred per-site.

It may also be useful to have a undercloud with using IPv4 DNS servers, while the provisioning network is IPv6. We then need a way to specify IPv6 DNS servers for the ctlplane subnets so that overcloud nodes with IPv6 addresses can resolve names.