Fig config design 649

Design sketch for issue #649 
A related case that's probably worth treating specially is the case where a config item that is valid for fig but which is not supported by flocker is included.  It would most likely be best if we reported those cases in a different way to the user - so they quickly get the idea that while they did a good job writing a fig config, flocker can't deal with it yet - rather than thinking flocker is buggy for saying their valid fig config is invalid.