Select an accessible image repository for some users

[laozc]

Now it's possible to fallback to a mirror site for those users
who cannot access the main k8s.gcr.io repository. You may also
set the --image-mirror-country to forcibly use a known local
mirror in the specific location.

[k8s-ci-robot]

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[minikube-bot]

Can one of the admins verify this patch?

[tstromberg]

@minikube-bot OK to test

[tstromberg]

Looks good so far, though I'll need to give this a more careful review. Thank you!

[tstromberg]

Since I like this change overall and want to see it merged, what do you think about supporting a flag, such as --image-repository=auto to trigger this logic? At least until we can figure out a way to either:

• Only auto-select mirrors for the country the user appears to be in
• Or Fetch Kubernetes images using tag@hash rather to prevent invalid downloads #4104 is resolved, where mirror auto-selection is guaranteed to be safe

[tstromberg]

Merging! I'm hoping to make "auto" the default once #4104 can be addressed. Thank you so much for your hard work here.