Do not assume all Knative installations use Istio

[bbrowning]

This is related to serverless/serverless-knative#2 but there are code changes required in both repos to fix this.

Not all Knative installations use Istio and even for the ones that do not all of them use the istio-ingressgateway service name, the istio-system namespace, nor do all of them give the Istio ingressgateway a public IP.

[rhuss]

Anything I can do to help to solve this issue ?

E.g. I managed to play through the Knative demo with minikube on my local machine, but broke at this point (because on minikube the istio setup is different, e.g. doesn't use a loadbalancer).

I think we can be much more flexible here and need to detect the api gateway in use, and react flexibly depending on the context.