HADOOP-15996.  Improved Kerberos username mapping strategy in Hadoop.
               Contributed by Bolke de Bruin

(cherry picked from commit d43af8b3db4743b4b240751b6f29de6c20cfd6e5)
(cherry picked from commit febafd0e4f6e7b0a95045565e149e3ed2bee4b6c)
13 files changed