Test dns_assignment with external dns integration

After the ports dns_assignment attribute calculation was changed by [1]
to implement [2], this patch updates the corresponding scenario test
case to reflect the current functionality

[1] https://review.opendev.org/c/openstack/neutron/+/731624/
[2] https://specs.openstack.org/openstack//neutron-specs/specs/victoria/port_dns_assignment.html

Change-Id: If667550a8745ea224d2099d662dd901dc5368ae5
Closes-Bug: #1956632
1 file changed