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

[hashibot]

This issue was originally opened by @devth as hashicorp/terraform#15004. It was migrated here as part of the provider split. The original body of the issue is below.

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This is somewhat related to #103

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

[ghost]

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