Merge pull request #574 from wallyworld/use-floatingip-as-public-address

Backport from trunk

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.