[QA] run tests with existing users

We need to make it possible to run tests with predefined users.

we can already use predefined users to run tests.
https://github.com/owncloud/ocis/blob/master/tests/acceptance/bootstrap/FeatureContext.php#L615-L703

but we still need an admin user. also the test will try to delete the users, we don't want that.

we need to modify the test code so that
1. run test without admin user
2. If we are running test with predefined user do not try to delete them.
3. after hooks should only delete resources that tests creates.