[SPARK-1870] Make spark-submit --jars work in yarn-cluster mode.

Sent secondary jars to distributed cache of all containers and add the cached jars to classpath before executors start. Tested on a YARN cluster (CDH-5.0).

`spark-submit --jars` also works in standalone server and `yarn-client`. Thanks for @andrewor14 for testing!

I removed "Doesn't work for drivers in standalone mode with "cluster" deploy mode." from `spark-submit`'s help message, though we haven't tested mesos yet.

CC: @dbtsai @sryza

Author: Xiangrui Meng <meng@databricks.com>

Closes #848 from mengxr/yarn-classpath and squashes the following commits:

23e7df4 [Xiangrui Meng] rename spark.jar to __spark__.jar and app.jar to __app__.jar to avoid confliction apped $CWD/ and $CWD/* to the classpath remove unused methods
a40f6ed [Xiangrui Meng] standalone -> cluster
65e04ad [Xiangrui Meng] update spark-submit help message and add a comment for yarn-client
11e5354 [Xiangrui Meng] minor changes
3e7e1c4 [Xiangrui Meng] use sparkConf instead of hadoop conf
dc3c825 [Xiangrui Meng] add secondary jars to classpath in yarn