[SPARK-2154] Schedule next Driver when one completes (standalone mode)

Author: Aaron Davidson <aaron@databricks.com>

Closes #1405 from aarondav/2154 and squashes the following commits:

24e9ef9 [Aaron Davidson] [SPARK-2154] Schedule next Driver when one completes (standalone mode)
(cherry picked from commit 9c249743eaabe5fc8d961c7aa581cc0197f6e950)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>