Update "Knative with Minikube" installation docs to use Gateway/VirtualService

[sslavic]

/area networking

/kind cleanup
/kind doc

knative-serving was migrated to Istio v1alpha3 Gateway / VirtualService via #1228.
Knative with Minikube installation docs need to be updated accordingly.

Notice, just like in current Knative with Minikube, Istio installation instructions include replacing LoadBalancer to NodePort in istio.yaml, similar instructions are likely needed for Knative installation step since now Knative release (and relase-lite.yaml) include knative-ingressgateway LoadBalancer type Service.

[tcnghia]

@sslavic I think LoadBalancer service runs fine on Minikube, and but would not receive an external IP address? And since LoadBalancer service is also a NodePort service it should receive node port correctly.

Thanks

[sslavic]

@tcnghia true. Then probably sed 's/LoadBalancer/NodePort/' is not needed for Istio installation instructions. /cc @trisberg