YARN-1101. Active nodes can be decremented below 0 (Robert Parker via tgraves)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1-beta@1518388 13f79535-47bb-0310-9956-ffa450edef68
YARN-981. Reverting on branch-2.1. to address build failures.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2.1-beta@1518413 13f79535-47bb-0310-9956-ffa450edef68