Run bats in gcp with ephemeral external ip addresses turned on

With the latest stemcell, when this is off we started seeing errors in syslog during a VM reboot like:
google-guest-agent.service: State stop-sigterm timed out
and the VM would hang for 10 minutes during the reboot.

ci/bats/iaas/gcp/prepare-bats-config.sh

@@ -66,7 +66,7 @@ properties:
         cloud_properties:
           network_name: "$(terraform_output "network")"
           subnetwork_name: "$(terraform_output "subnetwork")"
-          ephemeral_external_ip: false
+          ephemeral_external_ip: true
           tags: ["bosh-director"]
         dns: [8.8.8.8]
     - name: second
@@ -80,7 +80,7 @@ properties:
         cloud_properties:
           network_name: "$(terraform_output "network")"
           subnetwork_name: "$(terraform_output "second_subnetwork")"
-          ephemeral_external_ip: false
+          ephemeral_external_ip: true
           tags: ["bosh-director"]
         dns: [8.8.8.8]
 EOF