HDFS-4804. WARN when users set the block balanced preference percent below 0.5 or above 1.0. Contributed by Stephen Chu.


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1480099 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/branch-2' into branch-2-gd
Merge remote branch 'origin/branch-2-gd' into fb-MAPREDUCE-4884-branch-2-gd