title: Override dask scheduler_port
body: <p>I've tried several ports without success: 8787 is indeed busy serving rstudio. I could redirect rstudio, but shouldn't the following work?</p>
  
  <pre><code>from distributed import Client, LocalCluster
  cluster = LocalCluster( scheduler_port = 8785 , n_workers = 2 )
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>/home/ec2-user/anaconda3/lib/python3.6/site-packages/distributed/bokeh/core.py:56: UserWarning: 
  Port 8787 is already in use. 
  Perhaps you already have a cluster running?
  Hosting the diagnostics dashboard on a random port instead.
    warnings.warn('\n' + msg)
  tornado.application - ERROR - Multiple exceptions in yield list
  </code></pre>
  
  <p>...</p>
  
  <pre><code>~/anaconda3/lib/python3.6/multiprocessing/forkserver.py in connect_to_new_process(self, fds)
       64             raise ValueError('too many fds')
       65         with socket.socket(socket.AF_UNIX) as client:
  ---&gt; 66             client.connect(self._forkserver_address)
       67             parent_r, child_w = os.pipe()
       68             child_r, parent_w = os.pipe()
  
  ConnectionRefusedError: [Errno 111] Connection refused
  </code></pre>