[SPARK-2974] [SPARK-2975] Fix two bugs related to spark.local.dirs

This PR fixes two bugs related to `spark.local.dirs` and `SPARK_LOCAL_DIRS`, one where `Utils.getLocalDir()` might return an invalid directory (SPARK-2974) and another where the `SPARK_LOCAL_DIRS` override didn't affect the driver, which could cause problems when running tasks in local mode (SPARK-2975).

This patch fixes both issues: the new `Utils.getOrCreateLocalRootDirs(conf: SparkConf)` utility method manages the creation of local directories and handles the precedence among the different configuration options, so we should see the same behavior whether we're running in local mode or on a worker.

It's kind of a pain to mock out environment variables in tests (no easy way to mock System.getenv), so I added a `private[spark]` method to SparkConf for accessing environment variables (by default, it just delegates to System.getenv).  By subclassing SparkConf and overriding this method, we can mock out SPARK_LOCAL_DIRS in tests.

I also fixed a typo in PySpark where we used `SPARK_LOCAL_DIR` instead of `SPARK_LOCAL_DIRS` (I think this was technically innocuous, but it seemed worth fixing).
According to the "Running Spark on YARN" guide, SPARK_YARN_USER_ENV adds environment variables to the Spark processes launched through YARN.  Assuming that this is set in every YARN container that we use, can I just set a new environment variable in ExecutorRunnableUtil.prepareEnvironment() to record whether we're running in a YARN container?