title: Passing in Kerberos keytab/principal via SparkLauncher
body: <p><code>spark-submit</code> allows us to pass in Kerberos credentials via the <code>--keytab</code> and <code>--principal</code> options. If I try to add these via <code>addSparkArg("--keytab",keytab)</code> , I get a <code>'--keytab' does not expect a value</code> error - I presume this is due to lack of support as of v1.6.0. </p>
  
  <p>Is there another way by which I can submit my Spark job using this SparkLauncher class, with Kerberos credentials ? - I'm using Yarn with Secured HDFS.</p>