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
Yes, just encountered this being problematic as I was looking through code and thinking about #796. I can't even remember why I was `pop`ing it in the first place, some reason that's no longer relevant in the branch's most recent code anyway.