GetImageConfig should return default configs for the images

[jian-he]

Here we can return the default image, instead of return empty image

func (h *KubeDLHandler) GetImageConfig() *ImageConfig {
	cm := &v1.ConfigMap{}
	err := h.client.Get(context.Background(), types.NamespacedName{
		Namespace: constants.KubeDLSystemNamespace,
		Name:      constants.KubeDLConsoleConfig,
	}, cm)
	if err != nil {
		glog.Infof("get image config error: %v", err)
		return &ImageConfig{}
	}

[cbl315]

/assign

Just a suggestion, it would be better that label the issues like other projects.