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.

bin/spark-submit

@@ -35,8 +35,10 @@ while (($#)); do
   shift
 done
 
-if [ ! -z $DRIVER_MEMORY ] && [ ! -z $DEPLOY_MODE ] && [ $DEPLOY_MODE = "client" ]; then
-  export SPARK_MEM=$DRIVER_MEMORY
+DEPLOY_MODE=${DEPLOY_MODE:-"client"}
+
+if [ -n "$DRIVER_MEMORY" ] && [ $DEPLOY_MODE == "client" ]; then
+  export SPARK_DRIVER_MEMORY=$DRIVER_MEMORY
 fi
 
 $SPARK_HOME/bin/spark-class org.apache.spark.deploy.SparkSubmit "${ORIG_ARGS[@]}"