[FLOC-1045] Handle missing containers in DockerClient.list

Check for 404 exceptions when we call `inspect_container` in `DockerClient.list` because the container may have been removed by another thread.

Fixes https://clusterhq.atlassian.net/browse/FLOC-1045
It'd be nice if there was some helpers to keep this logic with the part above that creates the patch. (This comment isn't actionable).

Maybe:
``` python
patch = self.patch(flocker_docker_client._client, 'containers', simulate_missing_containers)
return flocker_docker_client.list().addBoth(always, lambda _: patch.restore())
```