SPARK-1652: Set driver memory correctly in spark-submit.

The previous check didn't account for the fact that the default
deploy mode is "client" unless otherwise specified. Also, this
sets the more narrowly defined SPARK_DRIVER_MEMORY instead of setting
SPARK_MEM.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #730 from pwendell/spark-submit and squashes the following commits:

430b98f [Patrick Wendell] Feedback from Aaron
e788edf [Patrick Wendell] Changes based on Aaron's feedback
f508146 [Patrick Wendell] SPARK-1652: Set driver memory correctly in spark-submit.