[SPARK-1395] Fix "local:" URI support in Yarn mode (again).

Recent changes ignored the fact that path may be defined with "local:"
URIs, which means they need to be explicitly added to the classpath
everywhere a remote process is started. This change fixes that by:

- Using the correct methods to add paths to the classpath
- Creating SparkConf settings for the Spark jar itself and for the
  user's jar
- Propagating those two settings to the remote processes where needed

This ensures that both in client and in cluster mode, the driver has
the necessary info to build the executor's classpath and have things
still work when they contain "local:" references.

The change also fixes some confusion in ClientBase about whether
to use SparkConf or system properties to propagate config options to
the driver and executors, by standardizing on using data held by
SparkConf.

On the cleanup front, I removed the hacky way that log4j configuration
was being propagated to handle the "local:" case. It's much more cleanly
(and generically) handled by using spark-submit arguments (--files to
upload a config file, or setting spark.executor.extraJavaOptions to pass
JVM arguments and use a local file).

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #560 from vanzin/yarn-local-2 and squashes the following commits:

4e7f066 [Marcelo Vanzin] Correctly propagate SPARK_JAVA_OPTS to driver/executor.
6a454ea [Marcelo Vanzin] Use constants for PWD in test.
6dd5943 [Marcelo Vanzin] Fix propagation of config options to driver / executor.
b2e377f [Marcelo Vanzin] Review feedback.
93c3f85 [Marcelo Vanzin] Fix ClassCastException in test.
e5c682d [Marcelo Vanzin] Fix cluster mode, restore SPARK_LOG4J_CONF.
1dfbb40 [Marcelo Vanzin] Add documentation for spark.yarn.jar.
bbdce05 [Marcelo Vanzin] [SPARK-1395] Fix "local:" URI support in Yarn mode (again).
SPARK-1528 - add support for accessing remote HDFS
review comments