[stable/cluster-autoscaler] Fix cluster autoscaler with default dns policy

[philipbjorge]

This adds the default dns policy to the cluster autoscaler as seen in kops -- kubernetes/kops#3395

Full discussion for the fix and bug can be seen here -- kubernetes/kops#1796

[k8s-ci-robot]

Hi @philipbjorge. Thanks for your PR.

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[viglesiasce]

/ok-to-test

[viglesiasce]

@philipbjorge should it always be Default even outside of Kops+aws? Can you make it configurable either way?

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