[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.


Actually, I think here you can just say the following
```
Same as `spark.yarn.driver.memoryOverhead`, but for the ApplicationMaster in client mode.
```