fig local_port

Include one extra pr (https://github.com/docker/fig/pull/396), which will hopefully be merged soon-ish.

Adds a new fig command: `fig local_port` for printing the local port binding.
We can also use this from python code:

```python
def get_service_port(service_name, port, number=0):
    cmd = command.Command()
    project = cmd.get_project(cmd.get_config_path())
    service = project.get_service(service_name)
    return service.get_container(number=number).get_local_port(port)
```
No, not really.  Without a mock this is going to hit a docker server.  `create_autospec()` mimics the interface of the object so this is actually better than a fake for unit testing.

I understand the concern about mocking when it comes to integration tests, but for unit tests mocking is exactly what you want (otherwise you aren't really testing a unit).