Univention Corporate Server 4.1 erratum 206 (bugfix)

This update addresses the following issue(s):
* Minor code changes make other projects easier that rely on the App
  Center.
* Uninstalling an App failed when the App was not properly registered before.
* Tests run before upgrading an App used the installed version, not the
  to-be-installed version.
* A function to detect a docker bridge network conflict has been added.
* The use of UDM handler objects has been optimized.