Comment 1 for bug 1667172

Looking in the target controller log we can see this error:
machine-0: 15:26:29 ERROR juju.provider.gce.gceapi while updating metadata for instance "juju-bb0d50-0" (juju-controller-uuid="49de7dbf-ac41-4160-8367-083503cf7125"): googleapi: Error 400: Invalid value 'https://
www.googleapis.com/compute/v1/projects/gothic-list-89514/zones/us-central1-a'. Values must match the following regular expression: '[a-z](?:[-a-z0-9]{0,61}[a-z0-9])?', invalidParameter

Passing the zone name without the URL (so "us-central1-a" in this case) works.