gke: Remove hardcoded default machine types

[bobbypage]

Remove hardcoded machine types and instead rely on the server side defaults. Only pass machineType flag if machine type is explicitly provided.

[k8s-ci-robot]

Hi @bobbypage. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[bobbypage]

Followup on @SergeyKanzhelev comment here - https://github.com/kubernetes-sigs/kubetest2/pull/171#pullrequestreview-797263709

[bobbypage]

/assign @amwat

[chizhg]

/ok-to-test /lgtm

[amwat]

/approve

Thank you!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: amwat, bobbypage, SergeyKanzhelev

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