cron triggers not executed

[Lars]

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.