[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.


by the way this check is incorrect. Right now we print this warning whenever the user runs YARN in client mode, regardless of whether the config is set. Also, we should do the correct check in `validateArgs` instead