Host must be cleared to avoid 404's

[nikkithurmond]

/area autoscale
/area networking
/kind bug
/kind dev

/assign @tcnghia

Expected Behavior

When the activator is in the Istio mesh, requests can be routed through the k8s service IP.

Actual Behavior

When the activator is in the Istio mesh, in order to route requests through the k8s service IP, the Host must be cleared to avoid 404's.

Steps to Reproduce the Problem

1. Once Enable Istio Sidecar injection for activator #833 is merged, comment out line 53 from cmd/ela-activator/main.go
   r.Host = ""
2. Enable scale to zero. Modify config/elaconfig.yaml. Set 'autoscale.enable-scale-to-zero' to 'true'
3. Replace the activator
   bazel run //config:activator.replace
4. Deploy an app, allow it to scale to zero.
5. Send a request to your app. Without clearing the host, you should receive a 404.

[tcnghia]

We will need to investigate this after moving to v1alpha3 in #1228 since the behavior may have changed a lot.