WIP: networker skeleton

worker: networker skeleton

This RP is a skeleton for the future networker based on StringsWorker

Continue of work from: https://github.com/juju/juju/pull/86
I'm pretty sure this has no business pulling the AptGet constant from cloudinit. Please put the constant somewhere it can be imported from both places. If that's a new package with a single constant, that's better than introducing a dependency between these packages in either direction.