As a Knative administrator, I want to list and set autoscaling configs which apply to overall Knative platform.

[zhanggbj]

A developer may set some autoscaling configs like --limits-cpu or --limits-memory as what kn service create described today. However, a Knative administrator may need to update some overall configs like stable-window or panic-window-percentage or ticker-interval for the whole Knative platform. The kn-admin will list all the config we support and set a detailed config.

$ kn admin autoscaling list
- Stable-window
- panic-window-percentage
- ticker-interval
…

$ kn admin autoscaling set --stable-window=xx --panic-window-percent=xx