Port range support in the firewaller.

The firewaller watches the openedPorts collection, the ports field on units is no longer updated.

This work fixes https://bugs.launchpad.net/juju-core/+bug/1337817
...so I'd expect to see `nil, err("pow")` here.

I think you can fix it easily by grabbing the authorizer at the start of the api method, instead of getting a fresh one for every entity.