Upcloud: Add possibility to setup cluster using nodes with no public IPs

[Xartos]

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What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change /kind bug /kind cleanup /kind design /kind documentation /kind failing-test /kind feature /kind flake

What this PR does / why we need it:

To increase the attack surface of clusters in upcloud we want to reduce the publicly facing IPs. Therefore this PR adds support for setting up a cluster without public IPs on the nodes. This was impossible previously.

Which issue(s) this PR fixes:

Fixes #N/A

Special notes for your reviewer:

This PR depends on this PR, so I'll wait with merging this one until that PR is merged.

This change will change the output variable names.

Does this PR introduce a user-facing change?:

terraform upcloud: Add possibility to setup cluster using nodes with no public IPs

[k8s-ci-robot]

Hi @Xartos. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[yankay]

/ok-to-test

[VannTen]

Not much comment (not my area of expertise), but I suggest you split your two last commits into a separate PR, since they are just linting fixes, that will make it easier to review (unless they fixes previous commits, in which case you should squash them into the approriate commits).

(Probably apply for other stuff as well, for examples, the schema removal in docs, agree it's redundant but it's probaly simpler to pull that into a separate change)