Code review comment for lp:~lifeless/pyjuju/bug-945505

So, it's a little perverse putting ip_address in an attribute named dns_name but it's what I do with the openstack provider too.

Having this as a flag-day revision rather than continuing with the old behaviour for previous txaws releases I'm a bit wary of, given the lack of a txaws release that actually includes the required change.

Updating juju/machine/__init__.py or at least the comment there would be good.