containerd: Set a default version even with Kubernetes 1.17

[hakman]

For testing I need to be able to enable the containerd container runtime with Kubernetes 1.17 without setting the version in cluster spec.
It is a tiny change that should have minimal effect outside e2e tests.

[k8s-ci-robot]

Hi @hakman. Thanks for your PR.

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[rifelpet]

/ok-to-test

[hakman]

/retest

[hakman]

/retest

[rifelpet]

Since this still requires users explicitly opt in to containerd which isnt in a stable kops release yet, this is a pretty safe change. Thanks!

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hakman, rifelpet

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