Changed find_by back to find_by_fqname

Issue was introduced in ManageIQ#2004, commit c281edf find_by_fqname was changed to use rails find_by but in this case fqname is not an actual column, find_by_fqname is an actual method in MiqAeNamespace class https://github.com/ManageIQ/manageiq/blob/cb5e0de1766fa363ecfd19715316e17053925d1e/app/models/miq_ae_namespace.rb#L20

https://bugzilla.redhat.com/show_bug.cgi?id=1510463

app/controllers/miq_ae_class_controller.rb

@@ -217,7 +217,7 @@ def open_parent_nodes(record)
         self.x_node = "#{selected_node[0]}-#{to_cid(record.id)}"
         parents.push(record)
       else
-        ns = MiqAeNamespace.find_by(:fqname => nodes[0..i].join("/"))
+        ns = MiqAeNamespace.find_by_fqname(nodes[0..i].join("/"))
         parents.push(ns) if ns
       end
     end