Teardown forwarding rule check not specific

[bgeesaman]

https://github.com/GoogleCloudPlatform/gke-istio-gce-demo/blob/master/scripts/teardown.sh#L59 When two sets of Istio clusters are running in a single project, the teardown script waits until "all forwarding rules are gone". However, the gcloud command lists all Istio related forwarding rules, including those that it isn't responsible for. So, it won't finish tearing down until the other cluster's forwarding rules are gone as well.