UI to accept arbitrary HPA yaml

[josephburnett]

The UI should accept a yaml object (e.g. HPA) which will be plumbed through the core all the way to the autoscaling plugin. So for an HPA plugin it would be a v2beta2 autoscaling HPA object. This allows the UI to configure the underlying autoscaling behavior in a natural way without the core having to couple to the details.

This should be part of the scenario interface (#62) and the plugin interface (#58).