Univention Corporate Server 4.4 erratum 19 (bugfix)

This update addresses the following issues:
* udm users/user create without --set username does not cause a exception
  anymore.
* Various LDAP filters are now escaped.
* The use of obsolete univention.admin.config has been removed.
* The computers/* UDM handlers now share a common base class. Code
  redundancy has therefore been reduced.
* The UMC now uses json instead of simplejson.
* It is now possible again to modify the LDAP base, for example reference
  policies.  This was broken since UCS 4.3-0.
* Default containers for domain controller computer objects can now be
  configured.