Merge -r 1428399:1428400 from trunk to branch-2. Fixes: YARN-301. Fair scheduler throws ConcurrentModificationException when iterating over app's priorities. Contributed by Sandy Ryza.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1428401 13f79535-47bb-0310-9956-ffa450edef68