cloud.gce.zone helps to retrieve instances running in a given zone. It should be one of the
GCE supported zones.
The GCE discovery can support multi zones although you need to be aware of network latency between zones.
To enable discovery across more than one zone, just enter add your zone list to
cloud: gce: project_id: <your-google-project-id> zone: ["<your-zone1>", "<your-zone2>"] discovery: zen.hosts_provider: gce