OAK-516 : Create LdapLoginModule based on ExternalLoginModule

- default synchandler should not sync the password into the repository
- ldaploginmodule does not populate subject during commit
- add test for re-login


git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1547765 13f79535-47bb-0310-9956-ffa450edef68