[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:
Item 1 `--am-memory MEM` in SparkSubmit args
Item 2 `spark.yarn.appMaster.memory` in SparkConf or System Properties
Item 3 `SPARK_YARN_AM_MEMORY` in System env
Item 4 default value 512m

Note: this arguments is only available in yarn-client mode and will be assigned with driver memory in yarn-cluster mode.