Support Additional Puppet Argument, e.g. --environment

[nnutter]

We have a workflow where we push development branches which get deployed as Puppet environments and I think I would need image_build to support specification of an --environment for development purposes.

[nnutter]

I've started trying to prototype this to see if it's possible.

P.S. Most of the changes are aesthetic and can be discarded if not desired.

[garethr]

@nnutter thanks, go open a PR when you have something you'd some extra eyes on. Happy to review or answer any questions you might have. This sounds like a useful feature.