130406 jvonka: REPO-1062 / MNT-16176: allow existing users to use SSO external auth via Sharewhen repo is read-only
- re-implement the fix (note: we do not allow auto-creation of missing users)
git-svn-id: https://svn.alfresco.com/repos/alfresco-enterprise/alfresco/HEAD/root@132128 c4b6b30b-aa2e-2d43-bbcb-ca4b014f7261