Automatic merge from submit-queue

Add ProviderUid support to Federated Ingress

This PR (along with GLBC support [here](https://github.com/kubernetes/ingress/pull/278)) is a proposed fix for #39989. The Ingress controller uses a configMap reconciliation process to ensure that all underlying ingresses agree on a unique UID. This works for all of GLBC's resources except firewalls which need their own cluster-unique UID. This PR introduces a ProviderUid which is maintained and synchronized cross-cluster much like the UID. We chose to derive the ProviderUid from the cluster name (via md5 hash).

Testing here is augmented to guarantee that configMaps are adequately propagated prior to Ingress creation.

```release-note
Federated Ingress over GCE no longer requires separate firewall rules to be created for each cluster to circumvent flapping firewall health checks.
```

cc @madhusudancs @quinton-hoole