✨ feat: OpenStackMachine: add OpenStackServerRef to Status

[EmilienM]

What this PR does / why we need it:

An OpenStackMachine object has now its OpenStackServerRef in Status
so we can know to with OpenStackServer it's attached to.

We will set that ref as soon as we found an existing object, whether the
server is running or not yet or in error state.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2401

[k8s-ci-robot]

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[netlify]

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

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[EmilienM]

/cherry-pick release-0.12

[k8s-infra-cherrypick-robot]

@EmilienM: once the present PR merges, I will cherry-pick it on top of release-0.12 in a new PR and assign it to you.

In response to this:

/cherry-pick release-0.12

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[lentzi90]

/lgtm
Thanks!

[k8s-ci-robot]

New changes are detected. LGTM label has been removed.

[EmilienM]

fixed lint, sorry

[k8s-ci-robot]

@EmilienM: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cluster-api-provider-openstack-test	a8b3931	link	true	/test pull-cluster-api-provider-openstack-test

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository. I understand the commands that are listed here.

[EmilienM]

/hold
This needs more discussion with @mdbooth