Implement a check to see if an ISO URL is valid

[Twister915]

Fixes #3282

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[minikube-bot]

Can one of the admins verify this patch?

[tstromberg]

Nice work! Just two minor comments.

[RA489]

/lgtm

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This pull-request has been approved by: RA489, Twister915

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/lgtm

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[tstromberg]

@minikube-bot OK to test

[Twister915]

I do not see anything I caused in the failing tests. Are there any required changes @tstromberg?