Instead of allowing the cloud provider to guess at the zones that
should be applied for a cluster under test, allow the explicit list
of zones to consider to be passed as a new test context flag -gce-zones.
Only the GCE test cloud provider recognizes this value because only
the GCE test cloud provider makes assumptions about zones for verifying
values, and the default assumptions for GKE do not always match non-GKE
providers.
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