Ensure floating IP is used as public address for openstack

Fixes https://bugs.launchpad.net/juju-core/+bug/1348287

If a floating IP address is requested for an Openstack instance, we know that will be the public address. That one should be reported as such instead of trying to guess. Often the guesses can be wrong.


On second thoughts, should we really be doing this if address's scope is not public? That would change internal address selection logic.