MAPREDUCE-5086. MR app master deletes staging dir when sent a reboot command from the RM. Contributed by Jian He


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1464255 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-YAHOO-276-trunk-gd