Add a system modeler to scheduler

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

pkg/client/cache/listers.go

@@ -52,6 +52,15 @@ func (s *StoreToPodLister) List(selector labels.Selector) (pods []api.Pod, err e
 	return pods, nil
 }
 
+// Exists returns true if a pod matching the namespace/name of the given pod exists in the store.
+func (s *StoreToPodLister) Exists(pod *api.Pod) (bool, error) {
+	_, exists, err := s.Store.Get(pod)
+	if err != nil {
+		return false, err
+	}
+	return exists, nil
+}
+
 // StoreToNodeLister makes a Store have the List method of the client.NodeInterface
 // The Store must contain (only) Nodes.
 type StoreToNodeLister struct {

pkg/client/cache/listers_test.go

@@ -72,5 +72,21 @@ func TestStoreToPodLister(t *testing.T) {
 			t.Errorf("Expected %v, got %v", e, a)
 			continue
 		}
+
+		exists, err := spl.Exists(&api.Pod{ObjectMeta: api.ObjectMeta{Name: id}})
+		if err != nil {
+			t.Errorf("unexpected error: %v", err)
+		}
+		if !exists {
+			t.Errorf("exists returned false for %v", id)
+		}
+	}
+
+	exists, err := spl.Exists(&api.Pod{ObjectMeta: api.ObjectMeta{Name: "qux"}})
+	if err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+	if exists {
+		t.Errorf("Unexpected pod exists")
 	}
 }