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.
Maybe it's sort of redundant, but we've seen enough people running different HDFS services under different Kerberos realms that I think it should be mentioned here that the user running the Spark job needs to be able to access all the listed NNs (either by them being on the same realm or in a trusted realm).

Also, nits: backquotes around the example, and capitalize "Spark" before application.