Ubernetes Lite GCE: Support multiple zones in GCE cloud provider

We adapt the existing code to work across all zones in a region.

We require a feature-flag to enable Ubernetes-Lite

Reasons:

* There are some behavioural changes if users create volumes with
the same name in two zones.
* We don't want to make one API call per zone if we're not running
Ubernetes-Lite.
* Ubernetes-Lite is still experimental.

There isn't a parallel flag implemented for AWS, because at the moment
there would be no behaviour changes from this.

hack/verify-flags/exceptions.txt

@@ -6,7 +6,7 @@ cluster/aws/templates/salt-minion.sh:# We set the hostname_override to the full
 cluster/centos/util.sh:  local node_ip=${node#*@}
 cluster/gce/configure-vm.sh:  advertise_address: '${EXTERNAL_IP}'
 cluster/gce/configure-vm.sh:  api_servers: '${KUBERNETES_MASTER_NAME}'
-cluster/gce/configure-vm.sh:  cloud_config: /etc/gce.conf
+cluster/gce/configure-vm.sh:  cloud_config: ${CLOUD_CONFIG}
 cluster/gce/configure-vm.sh:  runtime_config: '$(echo "$RUNTIME_CONFIG" | sed -e "s/'/''/g")'
 cluster/gce/coreos/helper.sh:# cloud_config yaml file should be passed
 cluster/gce/util.sh:    local node_ip=$(gcloud compute instances describe --project "${PROJECT}" --zone "${ZONE}" \