gce: use set type where order does not matter

[justinsb]

Starting small with FirewallRule Allowed and SourceRanges fields,
but this should be easier than explicitly handling values
where order does not matter.

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[justinsb]

(This isn't urgent, I think this is a nice-to-have to address this problem going forwards, we can discuss in office hours)

Note one (negative?) consequence of this is that the rules are string-sorted in terraform. I think that's a good thing going forwards, but it will cause a text-level diff in the terraform output (though not a cloud-level diff, because terraform also knows order doesn't matter here)

[hakman]

/retest

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@justinsb: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-kops-e2e-k8s-gce-cilium	ee0b15b	link	true	/test pull-kops-e2e-k8s-gce-cilium
pull-kops-e2e-k8s-gce-ipalias	ee0b15b	link	true	/test pull-kops-e2e-k8s-gce-ipalias

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