title: "ipcluster nbextension enable" doesn't have subcommand specified
body: <p>I have ipyparallel 5.3.0. Its official doc, <a href="https://github.com/ipython/ipyparallel#install" rel="nofollow">https://github.com/ipython/ipyparallel#install</a>, instructs the following:</p>
  
  <blockquote>
    <p>To enable the IPython Clusters tab in Jupyter Notebook:</p>
  </blockquote>
  
  <pre><code>ipcluster nbextension enable
  </code></pre>
  
  <p>But I got the following issue hence it didn't work.</p>
  
  <pre><code>No subcommand specified. Must specify one of: ['start', 'stop', 'engines'].
  </code></pre>
  
  <p>I did more checks following minrk's tips.</p>
  
  <pre><code>$ipcluster --version
  5.2.0
  
  $which -a ipcluster
  /home/etlolap/anaconda3/bin/ipcluster
  /user/bin/cluster
  
  $head -n 1 $(which ipcluster)
  #!/home/etlolap/anaconda3/bin/python
  </code></pre>
  
  <p>Everything looks fine though.</p>