Attempt to address off-cluster shell issue.

This is an attempt to address the issue that occurs when attempting to run a spark shell from outside a cluster. It's based on the chat in https://github.com/apache/incubator-spark/pull/192. Effectively this removes `spark_home` getting propagated to Workers by removing the parameter entirely from `ApplicationDescription` and any related invocations.

cc @Shopify/data-engineers for review please.