WIP: Port ranges provider support

Working on https://bugs.launchpad.net/juju-core/+bug/1337817 - enabling port range support in juju-core. This branch modifies the provider interface and its implementations to handle port ranges. They are silently converted into old-style port slices on the firewaller. Port range support in the firewaller is still in progress.
> I presume

Why would you do that? :)
(Domas checked with me, and AFAICT it is not possible with Azure; it's necessary to convert to individual ports.)