Fix building of model with a  scoped exclusion rule
Fix building of model with a  scoped exclusion rule

If we define an audit scope with a exclusion rule composed
with compute nodes, the compute model built is empty. I fix
the algo used to built this scoped compute model.

Change-Id: I675226c58474b7ea52b3f92e7b466adae40643b8
Closes-bug: #1658995

openstack/watcher,openstack/watcher,stackforge/watcher,stackforge/watcher