YARN-2841. Merging to branch-2.6 for hadoop-2.6.0-rc1.
YARN-2843. Merging to branch-2.6 for hadoop-2.6.0-rc1.
YARN-2794. Fixed log messages about distributing system-credentials. Contributed by Jian He.

(cherry picked from commit be7bf956e96dd0fd9b521ca71df9124b9cc5ebd0)