Required AM memory is "amMem", not "args.amMemory"

"ERROR yarn.Client: Required AM memory (1024) is above the max threshold (1048) of this cluster" appears if this code is not changed. obviously, 1024 is less than 1048, so change this

Author: derek ma <maji3@asiainfo-linkage.com>

Closes #1494 from maji2014/master and squashes the following commits:

b0f6640 [derek ma] Required AM memory is "amMem", not "args.amMemory"