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

...default and let application env override default options if applicable


I think SPARK_JAVA_OPTS should be obtained not only from application env, but also from worker's local settings. Sometimes different worker machines have different hardware configurations and we have to enable specific JVM options for certain workers instead of using the same settings for all. I suggest we use worker's local setting as default and let application settings override the worker's setting if applicable. 

Signed-off-by: shane-huang <shengsheng.huang@intel.com>