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

https://clusterhq.atlassian.net/browse/FLOC-1021
Should we also cover the case where inspect returns an empty RestartPolicy Name? I see that on my containers...

```
        "RestartPolicy": {
            "MaximumRetryCount": 0,
            "Name": ""
        },
```

...or do we assume that we only ever parse Flocker created containers which will always be configured with an explicit policy?