YARN-2826. Fixed user-groups mappings' refresh bug caused by YARN-2826. Contributed by Wangda Tan.

(cherry picked from commit df36edf751202db00d8f43103d7120ec56d70a04)
YARN-2505. Supported get/add/remove/change labels in RM REST API. Contributed by Craig Welch.

(cherry picked from commit 9a4e0d343e9e891c10ef6682e7b2231a59e69ade)