title: Spark: Determine the driver address from within a task
body: <p>For debugging purposes, I want to send a message from a spark task (running within an executor) to the driver machine.  The <a href="http://spark.apache.org/docs/latest/configuration.html#networking" rel="nofollow">docs</a> claim that <code>spark.driver.host</code> contains this information, but I don't know how to access it from worker (or from the driver, for that matter...)</p>