YARN-602. Fixed NodeManager to not let users override some mandatory environmental variables. Contributed by Kenji Kikushima.
svn merge --ignore-ancestry -c 1518077 ../../trunk/


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1518078 13f79535-47bb-0310-9956-ffa450edef68