Skip preload download if `--image-repository` is set

[chengyayu]

重现问题所需的命令：minikube start --cpus=2 --memory='2000mb' --vm-driver hyperkit --image-mirror-country='cn' --image-repository='registry.cn-hangzhou.aliyuncs.com/google_containers'

失败的命令的完整输出：

😄 minikube v1.9.2 on Darwin 10.15.4
✨ Using the hyperkit driver based on existing profile
✅ Using image repository registry.cn-hangzhou.aliyuncs.com/google_containers
👍 Starting control plane node m01 in cluster minikube
💾 Downloading Kubernetes v1.18.0 preload ...
> preloaded-images-k8s-v2-v1.18.0-docker-overlay2-amd64.tar.lz4: 131.59 MiB

minikube logs命令的输出：

使用的操作系统版本：macOS 10.15.4

[lmdkfs]

我也用到这个 版本了，preloaded image的时候特别慢，最后也失败了

[tstromberg]

Similarly to image downloads, the preload download is not currently available in China (both are hosted on Google).

We should probably disable the image preload download if a user has specified --image-repository so that it doesn't keep trying to attempt it.