Flocker deploy 121

I would like this to be looked over by someone, for guidance on what needs to be done, and confirmation I'm on the right track.

And, now, the comment on #121 that some of this might not be necessary for now.

There are a couple of inline "TODO"s I'm not sure about.

Fixes #121 
I think other errors will be handled in future tickets. Eg you'll probably implement a `Config.fromYamlFile` factory which will raise syntax errors, permissions errors etc, but all these will be caught in `parseArgs`, stringified and re-raised as `UsageErrors`...I think.