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 
Raise `twisted.python.usage.UsageError`, passing an error message argument.
That'll be caught by `ScriptRunner` and printed to stderr before exiting with a status code of 1.