Allow putting keytab file to HDFS location specified in spark.yarn.keytab

As a workaround for SPARK-19038 tried putting keytab in user's home directory in HDFS but this fails with

Exception in thread "main" org.apache.spark.SparkException: Keytab file: hdfs:///user/svc_odiprd/.kt does not exist
        at org.apache.spark.deploy.SparkSubmit$.prepareSubmitEnvironment(SparkSubmit.scala:555)
        at org.apache.spark.deploy.SparkSubmit$.submit(SparkSubmit.scala:158)
        at org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:124)
        at org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala)

This is yarn-client mode, so driver probably can't see HDFS while submitting a job; although I suspect it doesn't not only with yarn-client.

Would be great to support reading keytab for kerberos ticket renewals directly from HDFS.

We think that in some scenarios it's more secure than referencing a keytab from a local fs on a client machine that does a spark-submit.