[SPARK-4697][YARN]System properties should override environment variables

I found some arguments in yarn module take environment variables before system properties while the latter override the former in core module.
The method you're modifying (`getExtraClientArguments`) is the one that defines the `--name` argument for `ClientArguments`. And you're inverting the priority here, so that `spark.app.name` > `SPARK_YARN_APP_NAME`. So basically, since `spark.app.name` is mandatory, `SPARK_YARN_APP_NAME` becomes useless.

But please test it; make sure both work, both in client and cluster mode. Something might have changed since those fixes went in, although I kinda doubt it.