Fig flocker changestate yaml 796

Fixes #796
The implementation here converts Fig YAML -> Flocker YAML. A different implementation would be Fig YAML -> `Application` objects -> Flocker YAML. We already have Fig YAML -> `Application` objects implemented. Converting `Application` objects to Flocker YAML is also possible, albeit for the somewhat distinct use by `flocker-reportstate`.

The alternative implementation seem better as we'd only have one code path that generates Flocker YAML.