Allow users to configure the ELB Service to use with knative-sharedgateway

[tcnghia]

Expected Behavior

Users should be able to choose which ELB Service that the Gateway knative-sharedgateway uses. This should default to the Istio's istio-ingressgateway Service. Together with that we should point users to instruction of how to add a new ELB and use it, in case they want to reserve istio-ingressgateway for something else.

That we can stay out of the business of creating the ELB knative-ingressgateway in the Istio namespace, which is very much a leak of abstraction barrier.

Actual Behavior

We default to use the knative-ingressgateway.

Steps to Reproduce the Problem

Additional Info

[knative-prow-robot]

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In response to this:

Expected Behavior

Users should be able to choose which ELB Service that the Gateway knative-sharedgateway uses. This should default to the Istio's istio-ingressgateway Service. Together with that we should point users to instruction of how to add a new ELB and use it, in case they want to reserve istio-ingressgateway for something else.

That we can stay out of the business of creating the ELB knative-ingressgateway in the Istio namespace, which is very much a leak of abstraction barrier.

Actual Behavior

We default to use the knative-ingressgateway.

Steps to Reproduce the Problem

Additional Info

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[tcnghia]

/assign tcnghia

[tcnghia]

duplicate of #1969