YARN-2977. Fixed intermittent TestNMClient failure. (Contributed by Junping Du)

(cherry picked from commit cf7fe583d14ebb16fc1b6e29dc2afbf67d24b9d1)
CHANGES.txt: add YARN-2977

(cherry picked from commit 76b0370a27c482caff9498e15ef889d37f413ce7)

Conflicts:
	hadoop-yarn-project/CHANGES.txt