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
:question: A useful idiom for these cases is:

```
self._application_links.setdefault(application, []).append({...})
```

And then you don't need to initialize the list separately elsewhere.