Refactor validation logic around checking for multiple options

[justinsb]

We also have to move some of the EnvVar handling around to avoid a circular reference.

[justinsb]

WIP because I want to validate that this detects that the json names aren't yet right in v1alpha3 (also I need to rebase for kindnet)

[justinsb]

/retest

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/retest

[hakman]

/override pull-kops-e2e-k8s-gce-ipalias

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