Clean up and simplify Spark configuration

Over time as we've added more deployment modes, this have gotten a bit unwieldy with user-facing configuration options in Spark. Going forward we'll advise all users to run `spark-submit` to launch applications. This is a WIP patch but it makes the following improvements:

1. Improved `spark-env.sh.template` which was missing a lot of things users now set in that file.
2. Removes the shipping of SPARK_CLASSPATH, SPARK_JAVA_OPTS, and SPARK_LIBRARY_PATH to the executors on the cluster. This was an ugly hack. Instead it introduces config variables spark.executor.extraJavaOpts, spark.executor.extraLibraryPath, and spark.executor.extraClassPath.
3. Adds ability to set these same variables for the driver using `spark-submit`. 
4. Allows you to load system properties from a `spark-defaults.conf` file when running `spark-submit`. This will allow setting both SparkConf options and other system properties utilized by `spark-submit`.
5. Made `SPARK_LOCAL_IP` an environment variable rather than a SparkConf property. This is more consistent with it being set on each node.
Maybe the problem here lies with spark-ec2's default configuration setting SPARK_LOCAL_DIRS on the master when it should only really be used on workers, and in not setting `spark.local.dir`.

I think the current documentation for SPARK_LOCAL_DIRS sort of suggests that it acts as an override, without any caveats about whether it only should be used on workers, etc.