Merge -c 1576751 from trunk to branch-2 to fix YARN-1444. Fix CapacityScheduler to deal with cases where applications specify host/rack requests without off-switch request. Contributed by Wangda Tan.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1576752 13f79535-47bb-0310-9956-ffa450edef68