Update language on Ingress deployment page that specifically references GKE

We mention GKE even on a manually connected cluster, when we cannot get the IP address of the Ingress:

The IP address is in the process of being assigned. Please check your Kubernetes cluster or Quotas on Google Kubernetes Engine if it takes a long time. More information

Since we don't know that this cluster is actually on GKE, we probably should make a more generic statement or direct the user to a troubleshooting section in the docs.