[FLOC-1021] Implement restart policy in flocker.node._docker.

https://clusterhq.atlassian.net/browse/FLOC-1021
If #993 gets merged first, then this should be copied (not moved) to the `HostConfig` and supplied to `create_container_from _config`. If you don't also supply it to `start`, the configuration supplied to `create` gets clobbered.