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.


As discussed on IRC, the floating IP address is known to be public. If it were left for Juju to also guess that it is private, that may mask a true private address.