Make YARN client extend SparkApplication

For SPARK-11035 to work well, at least in cluster mode, YARN needs to implement SparkApplication so that it doesn't use system properties to propagate Spark configuration from spark-submit.

There is a second complication, that YARN uses system properties to propagate SPARK_YARN_MODE on top of other Spark configs. We should take a look at either change that to a configuration, or remove SPARK_YARN_MODE altogether if possible.