MAPREDUCE-5146. application classloader may be used too early to load classes. Contributed by Sangjin Lee.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1470694 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-xxx-hadoop-auth-kerberos-realm-trunk-gd