✨ [CP] Allow webhook changes to OpenStackCluster.Spec.Bastion

[tobiasgiese]

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Part of #1054
cherry-picks #1070

Special notes for your reviewer:

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

TODOs:

• squashed commits
• if necessary:
  • includes documentation
  • adds unit tests

/hold

_{Tobias Giese tobias.giese@daimler.com, Daimler TSS GmbH, legal info/Impressum}

[netlify]

✔️ Deploy Preview for kubernetes-sigs-cluster-api-openstack ready!

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😎 Browse the preview: https://deploy-preview-1075--kubernetes-sigs-cluster-api-openstack.netlify.app

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[tobiasgiese]

I got the wrong release branch 🙈