Multitenant autoscaler dynamic reconfiguration

[glyn]

Expected Behavior

Changes to the autoscaler or logging configuration should take effect without restarting the autoscaling controller.

Actual Behavior

Configuration is read when the autoscaling controller starts up and not subsequently.

Steps to Reproduce the Problem

1. Modify config/config-autoscaler.yaml so that scale-to-zero-threshold is "30s" rather than "5m".
2. Apply the change (e.g. ko apply -f config/config-autoscaler.yaml).
3. Apply load to a revision so it scales up from zero.
4. Observe that the revision takes 5 minutes to scale to zero rather than 30 seconds.

Additional Info

#1417 added dynamic reconfiguration to the single tenant autoscaler.

/area autoscale
/kind dev

[glyn]

/assign @glyn