OAK-1222 Migration of Group Members stored in tree structure

- refactor upgrade tests to get better test reports
- add group editor to manage migration of group memebers
- make logic in GroupMemembershipProvider reusable by upgrade

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1546337 13f79535-47bb-0310-9956-ffa450edef68