YARN-443. allow OS scheduling priority of NM to be different than the containers it launches (tgraves)

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1454413 13f79535-47bb-0310-9956-ffa450edef68
svn merge -c 1454460. Merging from trunk to branch-2 to fix HDFS-4567.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1454468 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd
Merge remote branch 'origin/branch-2-gd' into fb-YAHOO-242-branch-2-gd