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 controller is tightly coupled with the regular AWScluster and the EKS kinds.

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.

[1] https://github.com/kubernetes-sigs/cluster-api/blob/master/api/v1alpha4/cluster_types.go#L50-L58

Anything else you would like to add:
This can be overcame with something like #2124

Environment:

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

[enxebre]

cc @JoelSpeed @ncdc @randomvariable