If the code that https://github.com/crowbar/barclamp-quantum/pull/38 introduces is modified to be replaced with just:

quantums = search(:node, "roles:quantum-server AND roles:quantum-config-#{node[:nova][:quantum_instance]}") 
quantum = quantums.first || raise("Quantum instance '#{node[:nova][:quantum_instance]}' for nova not found")

That would elminate the need for these changes which seems a little hacky.
Does that proposed change make sense?