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
Instead of having one giant test covering all valid inputs, or at least in addition to, you should have a test per mini-parser. I.e. one for links, one for ports, etc..