Use all kops mirrors to determine artifacts hashes

[hakman]

Current implementation only looks for hash files in the S3 mirror and, if that is down, kops commands will fail with:

cannot determine hash for "https://kubeupv2.s3.amazonaws.com/kops/1.19.0-alpha.4/linux/amd64/nodeup"
(have you specified a valid file location?)

The code in the assets remapping and mirroring is quite complex and even if this looks a bit less elegant than I would like, it works pretty well.

Fixes: #9951

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[hakman]

/cc @justinsb @mikesplain @rifelpet @zetaab
/hold for comments

[zetaab]

/lgtm

[rifelpet]

This new logic and the constants look very similar to https://github.com/kubernetes/kops/blob/master/upup/pkg/fi/cloudup/urls.go would it make sense to consolidate them?

[hakman]

This new logic and the constants look very similar to https://github.com/kubernetes/kops/blob/master/upup/pkg/fi/cloudup/urls.go would it make sense to consolidate them?

It would make sense to move this to a function somewhere they both could use, but no idea where. It can also be done in a followup PR once we agree where to extract it.

[hakman]

/hold cancel

[zetaab]

/lgtm