Univention Corporate Server 4.0 erratum 55 (bugfix)

This update addresses the following issues:
* Check that joining account is member of "Domain Admins" in Active
  Directory before starting the Active Directory join
* Show warning if the SRV record _domainctonroller_master is missing.
* Allow UMC Administrator to be renamed before Active Directory join
* LDAP base DNs with uppercase letters caused synchronization rejects.