Fix access to Secure HDFS
Remove the runAsUser as it breaks secure hdfs access
Don't call the doAs if user is unknown or the same user that is already running
Merge pull request #149 from tgravescs/fixSecureHdfsAccess

Fix secure hdfs access for spark on yarn

https://github.com/apache/incubator-spark/pull/23 broke secure hdfs access. Not sure if it works with secure hdfs on standalone. Fixing it at least for spark on yarn.

The broadcasting of jobconf change also broke secure hdfs access as it didn't take into account things calling the getPartitions before sparkContext is initialized. The DAGScheduler does this as it tries to getShuffleMapStage.