Support set queue-proxy target port to app

[Leon-Liangming]

Expected Behavior

user can set app's port which queue-proxy request

Actual Behavior

queue-proxy send request to "http://localhost:8080", can't be set by user.
https://github.com/knative/serving/blob/master/cmd/queue/main.go#L233

target, err := url.Parse("http://localhost:8080")
if err != nil {
	logger.Fatal("Failed to parse localhost url", zap.Error(err))
}

1. user set a port name as "user-port" to service(knative)'s container port

2. revision controller set "user-port" as queue-proxy container's env "PORT"
   reference code:
   https://github.com/knative/serving/blob/master/pkg/reconciler/v1alpha1/revision/resources/deploy.go#L113-L135

3. queue-proxy get env "PORT"'s value, and replace the target address

Additional Info

[mattmoor]

This feels like one part of #2258.

[mdemirhan]

This is duplicate of #2258.