support customized java options for Master, Worker, Executor, and Repl

hi,
Sometimes I need to remote debug one of the processes of spark. Or set the heap dump path to diffrent locations for each process. So I need to set different java opts for each java process in spark. So I add some code in run , and the java options can be set in conf/spark-env.sh like this:
```
export SPARK_MASTER_OPTS='-Dspark_master'
export SPARK_WORKER_OPTS='-Dspark_worker'
export SPARK_EXECUTOR_OPTS='-Dspark_executor'
export SPARK_REPL_OPTS='-Dspark_repl'
```
Hope this can be merged into trunk.
Thanks.