Right size the activator

[akyyy]

/area autoscale
/kind dev

Expected Behavior

The activator replicas should be more than 1.

Actual Behavior

It's 1.

Additional Info

To avoid SPOF and dropping requests.

[mattmoor]

This issue should track more than simply making a 1 a 3. We need to figure out:

1. What the minimum footprint of the activator should be?
2. Should it be auto-scaled?

[markusthoemmes]

@akyyy should the title be "Right size the activator"?

[akyyy]

@markusthoemmes Fixed. thanks!

[mattmoor]

In addition to configuring HPA for the activator, we should include in "right sizing" making sure that we have placed reasonable resource limits on the activator pods.

See: https://kubernetes.io/docs/concepts/configuration/manage-compute-resources-container/#resource-requests-and-limits-of-pod-and-container