Pick up R executable from configuration first

For now, I can not run sparkr in cluster mode as livy pick up R executable from environment variable DRIVER_R, but spark didn't pass DRIVER_R to AM in yarn-cluster mode (This can be fixed in spark side). And since spark introduce spark.sparkr.r.command & spark.r.command for configuring R executable. Livy should also take this configuration over environment variable DRIVER_R.