[SPARK-1953][YARN]yarn client mode Application Master memory size is same as driver memory...

... size

Ways to set Application Master's memory on yarn-client mode:
1.  `spark.yarn.am.memory` in SparkConf or System Properties
2.  default value 512m

Note: this arguments is only available in yarn-client mode.


Hm... I agree that adding `clientmode` is clearer but it's also incredibly verbose. I think if we document this clearly that this is only for client mode then the user won't arbitrarily try to set it. I'll comment on #3409 about this as well.