Stop SparkListenerBus daemon thread when DAGScheduler is stopped.
Setting load defaults to true in executor
Merge pull request #392 from rxin/listenerbus

Stop SparkListenerBus daemon thread when DAGScheduler is stopped.

Otherwise this leads to hundreds of SparkListenerBus daemon threads in our unit tests (and also problematic if user applications launches multiple SparkContext).
Merge pull request #396 from pwendell/executor-env

Setting load defaults to true in executor

This preserves the behavior in earlier releases. If properties are set for the executors via `spark-env.sh` on the slaves, then they should take precedence over spark defaults. This is useful for if system administrators are setting properties for a standalone cluster, such as shuffle locations.

/cc @andrewor14 who initially reported this issue.