Read classpath entries from SPARK_EXTRA_CLASSPATH on executors. (#383)

This makes executors consistent with the driver. Note that
SPARK_EXTRA_CLASSPATH isn't set anywhere by Spark itself, but it's
primarily meant to be set by images that inherit from the base
driver/executor images.

Author: mccheah

resource-managers/kubernetes/docker-minimal-bundle/src/main/docker/executor/Dockerfile

@@ -27,5 +27,6 @@ COPY examples /opt/spark/examples
 CMD SPARK_CLASSPATH="${SPARK_HOME}/jars/*" && \
     if ! [ -z ${SPARK_MOUNTED_CLASSPATH}+x} ]; then SPARK_CLASSPATH="$SPARK_MOUNTED_CLASSPATH:$SPARK_CLASSPATH"; fi && \
     if ! [ -z ${SPARK_EXECUTOR_EXTRA_CLASSPATH+x} ]; then SPARK_CLASSPATH="$SPARK_EXECUTOR_EXTRA_CLASSPATH:$SPARK_CLASSPATH"; fi && \
+    if ! [ -z ${SPARK_EXTRA_CLASSPATH+x} ]; then SPARK_CLASSPATH="$SPARK_EXTRA_CLASSPATH:$SPARK_CLASSPATH"; fi && \
     if ! [ -z ${SPARK_MOUNTED_FILES_DIR} ]; then cp -R "$SPARK_MOUNTED_FILES_DIR/." .; fi && \
     exec /sbin/tini -- ${JAVA_HOME}/bin/java -Dspark.executor.port=$SPARK_EXECUTOR_PORT -Xms$SPARK_EXECUTOR_MEMORY -Xmx$SPARK_EXECUTOR_MEMORY -cp $SPARK_CLASSPATH org.apache.spark.executor.CoarseGrainedExecutorBackend --driver-url $SPARK_DRIVER_URL --executor-id $SPARK_EXECUTOR_ID --cores $SPARK_EXECUTOR_CORES --app-id $SPARK_APPLICATION_ID --hostname $SPARK_EXECUTOR_POD_IP