Note on how zk has to be up and running to migrate/clean zk

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1488005 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-HBASE-8559-trunk-gd