Track image storage usage for docker containers

add image fs info to summary stats API.
Adding node e2e test for image stats.

Signed-off-by: Vishnu kannan <vishnuk@google.com>

pkg/kubelet/container/testing/runtime_mock.go

@@ -137,3 +137,8 @@ func (r *Mock) GarbageCollect(gcPolicy ContainerGCPolicy) error {
 	args := r.Called(gcPolicy)
 	return args.Error(0)
 }
+
+func (r *Mock) ImageStats() (*ImageStats, error) {
+	args := r.Called()
+	return args.Get(0).(*ImageStats), args.Error(1)
+}