fix: validate taint effects

[immanuwell]

What this PR does / why we need it:

Reject invalid node taint effects in kOps validation.

Repro before this: put nvidia.com/gpu:ScheduleSometimes in spec.taints or spec.kubelet.taints. kOps accepted it, then the value could flow into kubelet taints / autoscaler tags. That effect is not a real Kubernetes node taint effect, so this is a tiny config footgun.

Now kOps accepts only NoSchedule, PreferNoSchedule, and NoExecute when an effect is set. Bad values get bounced early, nice.

Which issue(s) this PR fixes:

None found, searched issues/PRs for taint validation bits.

Special notes for your reviewer:

Tested with:

go test ./pkg/apis/kops/util ./pkg/apis/kops/validation ./pkg/model ./nodeup/pkg/model ./pkg/model/awsmodel ./pkg/model/azuremodel

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[k8s-ci-robot]

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