[SPARK-1953]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.  `--am-memory MEM` in SparkSubmit args
2.  `spark.yarn.appMaster.memory` in SparkConf or System Properties
3.  `SPARK_YARN_AM_MEMORY` in System env
4.  default value 512m

Note: this arguments is only available in yarn-client mode.


I am a little bit on the fence here about having this config in spark-submit.  I'm not sure if it will cause more confusion since it only applies to client mode.  I'm wondering if perhaps we just add the config for now.

@vanzin @andrewor14 thoughts on that since you both commented on the am.extraJavaOptions pr