SPARK-1528 - spark on yarn, add support for accessing remote HDFS

Add a config (spark.yarn.access.namenodes) to allow applications running on yarn to access other secure HDFS cluster.  User just specifies the namenodes of the other clusters and we get Tokens for those and ship them with the spark application.

Author: Thomas Graves <tgraves@apache.org>

Closes #1159 from tgravescs/spark-1528 and squashes the following commits:

ddbcd16 [Thomas Graves] review comments
0ac8501 [Thomas Graves] SPARK-1528 - add support for accessing remote HDFS