[release-0.9] 🐛 Fall back to cluster identityRef in absence of machine identityRef

[k8s-infra-cherrypick-robot]

This is an automated cherry-pick of #1768

/assign mdbooth

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mdbooth

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes-sigs/cluster-api-provider-openstack/blob/release-0.9/OWNERS)~~ [mdbooth] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[EmilienM]

/lgtm