Fig compatibility layer 649

TODO deal with port edge cases, e.g. just container port specified, or IP and port specified on host
See http://www.fig.sh/yml.html
Issues have been raised for this missing functionality, which is not yet supported by Flocker.

Fixes #649
Every logic branch needs a test... It would be simpler to just document "this method should only be called once" and then it's the caller's problem :grin: