Caches all pod delegates json for pods deletion without k8s info

This fixes #243 with following changes:
 + Optimize to fetch Pod from k8s client
 + Change to use cache always in DEL.
 + If failed to fetch the pod info from k8s clinet in deletion,
  use cached delegates as emergency bailout
 + Add test cases for cache

fixed conflicts in multus/multus.go & multus/multus_test.go

testing/testing.go

@@ -103,6 +103,11 @@ func (f *FakeKubeClient) AddPod(pod *v1.Pod) {
 	f.pods[key] = pod
 }
 
+func (f *FakeKubeClient) DeletePod(pod *v1.Pod) {
+	key := fmt.Sprintf("%s/%s", pod.ObjectMeta.Namespace, pod.ObjectMeta.Name)
+	delete(f.pods, key)
+}
+
 func NewFakePod(name string, netAnnotation string, defaultNetAnnotation string) *v1.Pod {
 	pod := &v1.Pod{
 		ObjectMeta: metav1.ObjectMeta{