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.
LGTM.

BTW, I know it's the current style of the file, but these long lines could use some breaks.