Univention Corporate Server 4.3 erratum 91 (bugfix)

This update addresses the following issue(s):
* Added "update-certificates" command to univention-appcenter.
* Join script now uses bindpwdfile options.
* When changing the App Center server, update all installed Apps, too.
* univention-app update now accepts --cache-dir to use a different directory
  for downloaded meta files.
* The deprecated AppManager class is not used anymore in the code.
* Categories were not translated. This has been fixed.
* Fixed permissions of cache directory.