title: Killing a SparkContext so I can create a new one
body: <p>Been trying to run a Jupyter Notebook setup for pyspark v2.1.1, but every time I try instantiating a context (freshly restarted kernel and derby.log file and metastore_db dir were deleted), I get the following error telling me a context is already running.</p>
  
  <pre><code>ValueError: Cannot run multiple SparkContexts at once; 
  existing SparkContext(app=PySparkShell, master=local[16]) created by 
  &lt;module&gt; at /home/ubuntu/anaconda2/lib/python2.7/site-packages/IPython/utils/py3compat.py:289
  </code></pre>
  
  <p>I've tried restarting the kernel and deleting the derby.log and also attempted to load that context by the app name and master it gives in the error, and then stop the context to no avail:</p>
  
  <pre><code>sc = SparkContext(app='PySparkShell', master='local[16]')
  sc.stop()
  </code></pre>
  
  <p>Has anyone had this problem and know how to just get a context running in a Jupyter Notebook when this happens?</p>