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
It's probably not necessary to repeat these notes again in this test. And perhaps these notes would be more useful if added to / merged  with the docstrings of the implementation...where they're more likely to be read.