Fix the dns-assignment on neutron port

https://bugs.launchpad.net/neutron/+bug/1873091

The dns-domain part of the dns-assignment for the neutron port is always taken from the dns-domain defined in the neutron configuration

Problem Description

When the user enable the dns or dns_domain_ports extension there are multiple ways to define Designate dns_domain (Designate zone) in Neutron side as follows:

• Neutron configuration file which contain the default value for dns_domain

• Neutron network definition which define the dns_domain for network and enable the dns extension for that network so every port created under the network will have a dns record in that dns_domain and it override the default value in configuration file

• Neutron port definition when the dns_domain_ports extension is enabled which is used to override the Neutron network dns_domain and create the dns record under another dns_domain

The dns-domain can be set on network level using openstack network set –dns-domain and on port level using dns_domain_ports extension openstack port create –dns-domain and both of those values (network level, port level dns-domians) are ignored when forming the port dns-assignment which always take the dns-domain in neutron config

Proposed Change

The port level dns-domain should take precedence over network level dns-domain which take precedence over the neutron config dns-domain which will be the default if the above two levels dont have dns-domain assigned

Testing

Unit Test

Unit test should be added to make sure that the right dns-domain is used in Neutron port dns assignment

Documentation Impact

User Documentation

The documemtation is showing the dns assignment with the dns domain defined in the Neutron configurations or defined in the Neutron network (which is usually the same) but not the dns_domain comming from Neutron port creare (use case 3) which should be updated

References

https://docs.openstack.org/neutron/latest/admin/config-dns-int-ext-serv.html#config-dns-int-ext-serv