Add SPARK_DIST_CLASSPATH to LAUNCH_CLASSPATH

## What changes were proposed in this pull request?
In Databricks, `SPARK_DIST_CLASSPATH` are used for driver classpath and `SPARK_JARS_DIR` is empty. So, we need to add `SPARK_DIST_CLASSPATH` to the `LAUNCH_CLASSPATH`. We cannot remove `SPARK_JARS_DIR` because Spark unit tests are actually using it.

Author: Yin Huai <yhuai@databricks.com>

Closes apache#50 from yhuai/Add-SPARK_DIST_CLASSPATH-toLAUNCH_CLASSPATH.

bin/spark-class

@@ -47,7 +47,7 @@ if [ ! -d "$SPARK_JARS_DIR" ] && [ -z "$SPARK_TESTING$SPARK_SQL_TESTING" ]; then
   echo "You need to build Spark with the target \"package\" before running this program." 1>&2
   exit 1
 else
-  LAUNCH_CLASSPATH="$SPARK_JARS_DIR/*"
+  LAUNCH_CLASSPATH="$SPARK_JARS_DIR/*:$SPARK_DIST_CLASSPATH"
 fi
 
 # Add the launcher build dir to the classpath if requested.