Change to use Yarn appropriate directories rather then /tmp or the user specified spark.local.dir
Throw exception if the yarn local dirs isn't set
Update docs and remove old reference to --user option
Allow for Executors to have different directories then the Spark Master for Yarn
Updated based on review comments.
Change Executor to only look at the env variable SPARK_YARN_MODE
Merge pull request #865 from tgravescs/fixtmpdir

Spark on Yarn should use yarn approved directories for spark.local.dir and tmp