Comment 3 for bug 1507568

So, here's what I think needs to happen, I just haven't had time to do it yet:

https://github.com/openstack/heat/blob/master/heat/engine/stack.py#L164

Whenever we handle a request where the request context project ID matches the stack stack_user_project_id, we need to disable evaluation of custom constraints (which I think is where the error is coming from above).