Fix import of docker.py

with the current code, instead of import docker from docker-py, it imports docker from the buildslave directory:

```
 
(sandbox)xavierd@xavierd-ThinkStation-E31:~/Projects/myGitHub/buildbot-work/masterPrototype$ twistd -b -y buildbot.tac
> /home/xavierd/Projects/myGitHub/buildbot-work/sandbox/local/lib/python2.7/site-packages/twisted/internet/base.py(1191)run()
-> self.startRunning(installSignalHandlers=installSignalHandlers)
(Pdb) c
> /home/xavierd/Projects/myGitHub/buildbot-work/src/master/buildbot/buildslave/docker.py(30)<module>()
-> client = None
(Pdb) l
 25  	    from docker import client
 26  	    _hush_pyflakes = [client]
 27  	except ImportError:
 28  	    import pdb
 29  	    pdb.set_trace()
 30  ->	    client = None
 31  	
 32  	
 33  	class DockerLatentBuildSlave(AbstractLatentBuildSlave):
 34  	    instance = None
 35  	
(Pdb) import docker
(Pdb) from docker import client
*** ImportError: cannot import name client
(Pdb) p docker
<module 'buildbot.buildslave.docker' from '/home/xavierd/Projects/myGitHub/buildbot-work/src/master/buildbot/buildslave/docker.py'>
(Pdb) 
```

docker.py should be renamed. I choose to prefix the file with b (for buildbot).

Note: there's the same error for the libvirt support in buildbot.