HOTFIX: Fix startup error when autogroups is not installed

This is the fix for issue #1219 (closed). It has two parts:

1. Extension of tox tests to enable testing of this bug
2. Bugfix

Our current tox config is testing AA always with all apps and services installed. Therefore it can not fully detect bugs that would only occur when some apps are not installed (e.g. auto groups not installed). The solution is to extend the tox config so that tests can run in one of these two modes:

• all: runs tests with all apps and services installed (as before)
• core: runs tests with only core packages installed, but no apps and services

e.g. tox -e py36-all will run tests with all apps installed. and tox -e py36-core will only run tests with only core apps installed.

I can confirm that the new core tests will detect the bug (with the same error occurring as reported in the issue) and that it therefore works as verification for this fix.

However, there is new problem: It is currently not possible to run all existing tests for core only due to existing cross-dependencies in several test cases, e.g. with auto groups. The current core tests therefore only consist of one test class and are basically just a startup test (which is sufficient to detect this bug).

Nevertheless I would suggest to adopt the existing test cases so that they can run with core only. This adoption would also be necessary if we ever wanted to care out existing apps btw.