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

https://clusterhq.atlassian.net/browse/FLOC-1021
I tested this by running the tests with an empty DockerClient namespace and got

```
[ERROR]
Traceback (most recent call last):
  File "/home/vagrant/.virtualenvs/1021/lib/python2.7/site-packages/twisted/python/threadpool.py", line 196, in _worker
    result = context.call(ctx, function, *args, **kwargs)
  File "/home/vagrant/.virtualenvs/1021/lib/python2.7/site-packages/twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/home/vagrant/.virtualenvs/1021/lib/python2.7/site-packages/twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
  File "/vagrant/flocker/node/_docker.py", line 521, in _list
    data[U"HostConfig"][u"RestartPolicy"])
  File "/vagrant/flocker/node/_docker.py", line 360, in _parse_restart_policy
    raise ValueError("Unknown restart policy: %r" % (data[u"Name"],))
exceptions.ValueError: Unknown restart policy: u''

flocker.node.functional.test_docker.GenericDockerClientTests.test_restart_policy_on_failure_maximum_count
-------------------------------------------------------------------------------
Ran 1 tests in 1.528s

```

```
diff --git a/flocker/node/functional/test_docker.py b/flocker/node/functional/test_docker.py
index 5d8700f..2a16aa5 100644
--- a/flocker/node/functional/test_docker.py
+++ b/flocker/node/functional/test_docker.py
@@ -545,11 +545,13 @@ CMD sh -c "trap \"\" 2; sleep 3"
         retry count is not restarted if it fails as many times than the
         specified maximum.
         """
+        self.namespacing_prefix = ''
         d = self.start_restart_policy_container(
             mode=u"failure",
             restart_policy=RestartOnFailure(maximum_retry_count=5))
 
         d.addCallback(self.assertEqual, "5")
+        return d

```