Enable autoscale for Istio gateways in E2E tests

[JRBANCEL]

Historically, the Istio Ingress Gateway replica count was fixed to 1 because there was no mechanism to figure out if a replica had been updated to the latest configuration. The workaround was to way until the 404 disappear and assume that at that point the replica had the desired configuration. This only work when there is a single replica.

#4734 addresses this. This is the logical next step after #4976.

Proposed Changes

• Enable HPA for Istio gateways in E2E tests

Next

• Reduce Istio's gateways resources quotas

[JRBANCEL]

/test pull-knative-serving-istio-1.1-mesh
/test pull-knative-serving-istio-1.1-no-mesh
/test pull-knative-serving-istio-1.2-mesh
/test pull-knative-serving-istio-1.2-no-mesh

[JRBANCEL]

TestTargetBurstCapacity failed and is known to be flacky.
/retest

[tcnghia]

/lgtm
/approve

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: JRBANCEL, tcnghia

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