Univention Corporate Server 4.1 erratum 53 (bugfix)

This update addresses the following issues:
* Support the new AWS API for EC2 region eu-central-1 (Frankfurt am Main).
* Add support for Virtual Private Clouds (VPC) with AWS EC2.
* The deprecated support for TLS/SSL connections and authentication through
  PAM has been removed.
* The deprecated code for booting old VMs through PXE with old qemu-0.x has
  been removed.
* The command line tool for debugging UVMM has been improved.
* Some unused helper code has been removed to fix an import error of the
  Python libvirt module from the UVMM Univention Directory Listener module
  handling the dynamic addition and removal of virtualization hosts.