refactor: create abstraction over cluster/instancegroup for building assets

[justinsb]

This abstraction should let us change the version on an instance group level.

[justinsb]

I think we need this one also, it introduces the idea that the kubernetes version is a property of the instance group, without needing to fully wire up the field.

cc @hakman

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[hakman]

/override pull-kops-e2e-cni-cilium-eni
/override pull-kops-e2e-cni-cilium-etcd

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/override pull-kops-e2e-cni-cilium-eni
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