[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.


minor point, but when I refactored this I made sure to keep these three things (parseArgs ... validateArgs) right after each other so it's easy to follow what the order is. I would move `val isClusterMode` after `validateArgs`