cron triggers not executed

I'm having problems with cron triggers in openstack stein release. None of my cron triggers get executed since the update.

It looks like the ThreadGroup doesn't get executed on line: https://github.com/openstack/mistral/blob/master/mistral/services/periodic.py#L166

If I directly call the run_periodic_tasks function, it gets executed once.

The same problem I can see in devstack on the master branch.