[FLOC-727] Design sketch of restart-policy handling.

https://clusterhq.atlassian.net/browse/FLOC-727
Maybe `RestartPolicy` is actually an interface and there are a few concrete implementations.  The primary method on the interface would be `serialize_to_docker_api_dictionary()`.  Concrete implementations would be `NeverRestart`, `OnFailureRestart`, and `AlwaysRestart`.  They could have whatever other attributes make sense (I think there's also a "what error codes count as failures" configuration for `on-failure`).  The config loader would need some way to construct the right objects but that shouldn't be hard.