Decouple Infrastructure resource from machine controller

[enxebre]

/kind feature

Describe the solution you'd like
As a CAPI consumer I'd like to plug-in my own ControlPlane and Infrastructure resources [1] while still still reusing the existing machine controller implementation.

Today this is not possible because the machine/machinePool controllers are tightly coupled with the regular AzureCluster kind [2].

A scenario where this is handy is one where there's a common vision for a controlPlane across providers e.g cluster-api-provider-nested and the infrastructure management can differ arbitrarily from the core implementations, e.g BYO.

[1] https://github.com/kubernetes-sigs/cluster-api/blob/master/api/v1alpha4/cluster_types.go#L50-L58
[2] https://github.com/kubernetes-sigs/cluster-api-provider-azure/blob/master/controllers/azuremachine_controller.go#L186-L204

Anything else you would like to add:
Analogous to AWS
kubernetes-sigs/cluster-api-provider-aws#2125
kubernetes-sigs/cluster-api-provider-aws#2124

Environment:

• cluster-api-provider-azure version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

[CecileRobertMichon]

As discussed in Slack, let's start with a proposal.

/milestone v0.5.x

[fejta-bot]

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Mark the issue as fresh with /remove-lifecycle stale.
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Send feedback to sig-contributor-experience at kubernetes/community.
/lifecycle stale

[CecileRobertMichon]

/remove lifecycle-frozen

Proposal kubernetes-sigs/cluster-api#4135

[CecileRobertMichon]

/close

done in #1389

[k8s-ci-robot]

@CecileRobertMichon: Closing this issue.

In response to this:

/close

done in #1389

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