Preparing to release hadoop-2.3.0

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1567112 13f79535-47bb-0310-9956-ffa450edef68
Release notes for hadoop-2.3.0.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1567119 13f79535-47bb-0310-9956-ffa450edef68