Comment 2 for bug 1657305

According to John Meinel:
'The charms should be updated to use "network-get <bindname>
--preferred-address" instead of just "unit-get private-address".
unit-get doesn't pass the information to Juju for us to know which bit
of the configuration we're supposed to be reporting.'

I would make the argument that private-address *should* be
predictable. It *should* be the PXE boot IP if not configurable.
As expressed in this bug:
https://bugs.launchpad.net/juju/+bug/1591962

These two bugs express the same issue. They say they are fix-released
but the fixes only deal with the symptom not the underlying problem.
https://bugs.launchpad.net/juju/+bug/1616098/
https://bugs.launchpad.net/juju/+bug/1603473/