Code review comment for ~mpontillo/maas:default-dns-ip--bug-1776604

One last comment. I think it's difficult to add more tests to this file, because effectively this function is a heuristic. It tries to guess the best interface to assign the node's primary hostname to. In cases where it guesses wrong, it's possible that customers have come to rely on the previous behavior. For example, it looks like we may be missing cases where there the PXE interface is configured with a triple-parent, such as:

physical (eth0) -> bond ({eth0, eth1} via bond0) -> vlan (bond0.11) -> bridge (br-vlan11)

In this case it's possible that a customer might be relying on MAAS to return an IP address assigned to bond0, when in fact we /could/ have a good argument that we should change the logic to return an IP address assigned to br-vlan11.