[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.


we should make the name of this consistent with what we decide for naming in https://github.com/apache/spark/pull/3409 (ie spark.yarn.clientmode.am...)