Upgrade a GKE cluster from single zone to multi zone without recreating it

[devth]

This is somewhat related to #15002

The gcloud CLI provides a way to convert a single-zone cluster to multizone.

In Terraform perhaps adding zones to additional_zones could use this API to non-destructively add a zone to the cluster. /cc @danawillow

[danawillow]

FYI this was fixed in hashicorp/terraform-provider-google#152

[devth]

@danawillow awesome thanks!

[ghost]

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