sort out docker workers

[tardyp]

Need to look at ​#2213, and see if there is anything to be done for docker network config as per discussion in:

Change default value for networking_config ​#2213

[tardyp]

So I can reproduce issue:

2016-06-27 18:46:45+0200 [-] while substantiating
        Traceback (most recent call last):
          File "/data/buildbot/sandbox/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 434, in errback
            self._startRunCallbacks(fail)
          File "/data/buildbot/sandbox/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 501, in _startRunCallbacks
            self._runCallbacks()
          File "/data/buildbot/sandbox/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 588, in _runCallbacks
            current.result = callback(current.result, *args, **kw)
          File "/data/buildbot/sandbox/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1184, in gotResult
            _inlineCallbacks(r, g, deferred)
        --- <exception caught here> ---
          File "/data/buildbot/sandbox/local/lib/python2.7/site-packages/twisted/internet/defer.py", line 1126, in _inlineCallbacks
            result = result.throwExceptionIntoGenerator(g)
          File "/data/buildbot/sandbox/local/lib/python2.7/site-packages/twisted/python/failure.py", line 389, in throwExceptionIntoGenerator
            return g.throw(self.type, self.value, self.tb)
          File "/data/buildbot/buildbot/master/buildbot/worker/docker.py", line 143, in start_instance
            res = yield threads.deferToThread(self._thd_start_instance, image, volumes)
          File "/data/buildbot/sandbox/local/lib/python2.7/site-packages/twisted/python/threadpool.py", line 246, in inContext
            result = inContext.theWork()
          File "/data/buildbot/sandbox/local/lib/python2.7/site-packages/twisted/python/threadpool.py", line 262, in <lambda>
            inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
          File "/data/buildbot/sandbox/local/lib/python2.7/site-packages/twisted/python/context.py", line 118, in callWithContext
            return self.currentContext().callWithContext(ctx, func, *args, **kw)
          File "/data/buildbot/sandbox/local/lib/python2.7/site-packages/twisted/python/context.py", line 81, in callWithContext
            return func(*args,**kw)
          File "/data/buildbot/buildbot/master/buildbot/worker/docker.py", line 189, in _thd_start_instance
            networking_config=self.networking_config
          File "/data/buildbot/sandbox/local/lib/python2.7/site-packages/docker/api/container.py", line 120, in create_container
            return self.create_container_from_config(config, name)
          File "/data/buildbot/sandbox/local/lib/python2.7/site-packages/docker/api/container.py", line 131, in create_container_from_config
            return self._result(res, True)
          File "/data/buildbot/sandbox/local/lib/python2.7/site-packages/docker/client.py", line 158, in _result
            self._raise_for_status(response)
          File "/data/buildbot/sandbox/local/lib/python2.7/site-packages/docker/client.py", line 154, in _raise_for_status
            raise errors.APIError(e, response, explanation=explanation)
        docker.errors.APIError: 500 Server Error: Internal Server Error ("json: cannot unmarshal string into Go value of type network.NetworkingConfig")

[tardyp]

​https://github.com/buildbot/buildbot/pull/2296