For standalone mode, add worker local env setting of SPARK_JAVA_OPTS as default and let application env override default options if applicable

Signed-off-by: shane-huang <shengsheng.huang@intel.com>
add comment in spark-env.sh.template for SPARK_JAVA_OPTS

Signed-off-by: shane-huang <shengsheng.huang@intel.com>
Merge pull request #788 from shane-huang/sparkjavaopts

For standalone mode, add worker local env setting of SPARK_JAVA_OPTS as ...