MAPREDUCE-5689. MRAppMaster does not preempt reducers when scheduled maps cannot be fulfilled. (lohit via kasha)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1555162 13f79535-47bb-0310-9956-ffa450edef68
HDFS-5695. Merge change r1555164 from trunk.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1555166 13f79535-47bb-0310-9956-ffa450edef68