This is to avoid unnecessary GCE API calls done by getInstanceByName
helper, which is iterating over all zones to find in which zone the
VM exists.
ProviderID already contains all the information - it's in the form:
gce://<VM URL> (VM URL contains project, zone, VM name).
ProviderID is propagated by Kubelet on node registration and in case
of bugs backfilled by node-controller.
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