Floating ip address updates

Fixes: https://bugs.launchpad.net/bugs/1364419

This is the second attempt to fix this issue. The first PR added code to use the floating IP address but it turns out that was only called at bootstrap, and then later the address poller overwrote the addresses. So extra code was needed in the instance gathering to also update the floating address details.
I don't think this is the appropriate place to do this. As it is, `AllInstances` will still return instances without the floating IP set. It should be the responsibility of `Addresses` to compute this, I think. Alternatively, make sure *everything* that is capable of creating an openstackInstance sets this field.