Preparing for hadoop-2.2.0 release.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1529752 13f79535-47bb-0310-9956-ffa450edef68
HDFS-4817. Moving changelog to Release 2.2.0 section to reflect the backport.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1529753 13f79535-47bb-0310-9956-ffa450edef68
Release notes for hadoop-2.2.0.

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/branch-2@1529757 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