YARN-1120. Made ApplicationConstants.Environment.USER definition OS neutral as the corresponding value is now set correctly end-to-end. Contributed by Chuan Liu.
svn merge --ignore-ancestry -c 1519330 ../../trunk/


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1519331 13f79535-47bb-0310-9956-ffa450edef68