Univention Corporate Server 4.2 erratum 151 (bugfix)

This update addresses the following issues:
* The temporary APT sources.list used for release updates is now removed if
  any of the pre-update scripts signals an abort condition.
* The check for QEMU virtual machines has been removed from the pre-update
  script.
* App updates that merely update software packages inside the App's Docker
  container are not displayed anymore as these updates are currently not
  supported through the App Center module.