title: stop all existing spark contexts
body: <p>I am trying to create a new Spark context using pyspark, and i get the following:</p>
  
  <blockquote>
    <p>WARN SparkContext: Another SparkContext is being constructed (or threw
    an exception in its constructor).  This may indicate an error, since
    only one SparkContext may be running in this JVM (see SPARK-2243). The
    other SparkContext was created at:
    org.apache.spark.api.java.JavaSparkContext.(JavaSparkContext.scala:61)</p>
  </blockquote>
  
  <p>I do not have any other context active (in theory), but maybe it did not finish correctly and it is still there. How can I find out if there is other or kill all the current ones? I am using spark 1.5.1 </p>