[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.


would be nice to add comment to spark.yarn.driver.memoryOverhead saying it applies in cluster mode.

This config is a bit different from the others as the memory overhead is purely a yarn thing and doesn't apply in other modes. ie There is no existing spark.driver.memoryOverhead.  We could potentially just use one config for this. I'm not sure if that will be more confusing or not though... @sryza @vanzin @andrewor14  thoughts on that?