Add a system modeler to scheduler

So it can try to predict the effect its bindings will have.

pkg/scheduler/predicates.go

@@ -342,6 +342,15 @@ func MapPodsToMachines(lister PodLister) (map[string][]api.Pod, error) {
 		return map[string][]api.Pod{}, err
 	}
 	for _, scheduledPod := range pods {
+		// TODO: switch to Spec.Host! There was some confusion previously
+		//       about whether components should judge a pod's location
+		//       based on spec.Host or status.Host. It has been decided that
+		//       spec.Host is the canonical location of the pod. Status.Host
+		//       will either be removed, be a copy, or in theory it could be
+		//       used as a signal that kubelet has agreed to run the pod.
+		//
+		//       This could be fixed now, but just requires someone to try it
+		//       and verify that e2e still passes.
 		host := scheduledPod.Status.Host
 		machineToPods[host] = append(machineToPods[host], scheduledPod)
 	}