YARN-2709. Made timeline client getDelegationToken API retry if ConnectException happens. Contributed by Li Lu.

(cherry picked from commit b2942762d7f76d510ece5621c71116346a6b12f6)
YARN-2715. Fixed ResourceManager to respect common configurations for proxy users/groups beyond just the YARN level config. Contributed by Zhijie Shen.

(cherry picked from commit c0e034336c85296be6f549d88d137fb2b2b79a15)