Fig flocker changestate yaml 796

Fixes #796
That sounds good, yes.  I'm confused though, it looks to me like the implementation already basically goes from Fig YAML to Application objects to Flocker YAML.  Isn't `application` in the method below an `Application` instance?  And isn't `self._applications` (being iterated over in `to_flocker_yaml` below` a `dict` of `Application` instances?

Possibly it would make sense for the `Application` -> Flocker YAML implementation to be somewhere other than on `FigConfiguration`, but apart from that the structure here seems okay.