HADOOP-11194. Ignore .keep files (kasha)

(cherry picked from commit d5084b9fa30771bffb03f2bad69304141c6e4303)
YARN-2673. Made timeline client put APIs retry if ConnectException happens. Contributed by Li Lu.

(cherry picked from commit 89427419a3c5eaab0f73bae98d675979b9efab5f)
Merge remote-tracking branch 'remotes/origin/branch-2' into branch-2