refactor(hetzner): refactored placement group code

[lukasmetzner]

What type of PR is this?

/kind cleanup

Optionally add one or more of the following kinds if applicable:

What this PR does / why we need it:

We have received some external contributions, which we want to clean up a little.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: lukasmetzner
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[k8s-ci-robot]

Hi @lukasmetzner. Thanks for your PR.

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