Comment 5 for bug 1307445

Michael, please see comments in bug 1435283 which iiuc is the same issue (so essentially a duplicate). Comment #11 explains one possible solution to the problem. Essentially what we want is for Juju to pick a port/interface on unit start and remember it (e.g. by uuid) so that if extra ports/interfaces are added to that unit, the private address should always be derived from that remembered port/interface. Shout or ping me in irc (dosaboy) if you want more clarification.