[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'd prefer not to add this to SparkSubmit. I've never seen someone have to fiddle with that value, so my guess is that this is such an uncommon need that those who want to use it wouldn't be bothered by the more verbose "--conf" approach.

Also, should probably add a "memory overhead" config too.