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The firewaller watches the openedPorts collection, the ports field on units is no longer updated.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
why are you stringifying this, doesn't that defeat the point of passing it via NewUnitTag ?

Also NewUnitTag can panic portRange.UnitName is not a valid Unit Tag, and if it was a value unit tag, it would already be a UnitTag, not a string.