Comment 3 for bug 1401929

The reason is the glance.image custom constraint here:

https://github.com/openstack/heat/blob/master/heat/engine/resources/server.py#L107

Probably the easiest quick fix is to skip the validation when count=0 - the behaviour introduced in the commit referenced as breaking this above is still desirable, as we'd want to fail fast on update in the event count > 0 is specified with some invalid data in the nested template.