include gke zone in list of extra locations

[mantis-toboggan-md]

Summary

Fixes #12261
Fixes #12260

Occurred changes and/or fixed issues

updates the GKE provisioning form so that when a user selects a zone, the locations field (which otherwise maps to the list of extra zones) includes that zone.

Areas or cases that should be tested

1. Create a 'zonal' GKE cluster without adding zones (default configuration), wait for the cluster to become active
2. Once the cluster has finished provisioning verify that you can edit it and add extra zones (gkeConfig.locations in the save request), and that the cluster is eventually 'active' again
3. Create a new 'zonal' gke cluster and define extra zones initially
4. Wait for the cluster to provision; verify that you can add/remove extra zones

Screenshot/Video

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