Handle "cluster.x-k8s.io/managed-by" annotation

[sbueringer]

/kind feature

Describe the solution you'd like
CAPI v1alpha4 introduced a mechanism to manage the cluster resource externally. This is triggered by the managed-by annotation. For more context, please see:

• 📖 CAEP: Add support for infrastructure cluster resources to be managed externally cluster-api#4135
• ✨ Add externally managed annotation and predicate cluster-api#4303

Anything else you would like to add:
Notes:

• We should make sure we do all that's necessary. I'm not entirely sure if it's enough to just ignore the cluster when the annotation is set. Please read the proposal for details
• Prereq: we need a rebase onto current CAPI before

[sbueringer]

/help

[k8s-ci-robot]

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[jichenjc]

@sbueringer just to confirm, this need v1alpha4 land first, correct?

[sbueringer]

@jichenjc No it just needs a rebase onto the current CAPI branch (i.e. use latest CAPI dependencies) (which I'll do later today)

[chrischdi]

Also relevant discussion maybe: https://kubernetes.slack.com/archives/C8TSNPY4T/p1620749340371300