Fix IScalingGroup.create_webhooks

Ah! Thank you, that makes so much more sense :)

So I guess the correct terminology would be that this method creates a webhook, and then document that it will return some information about that webhook.

How do you get the capability URL? In [the schema](https://github.com/rackerlabs/otter/blob/master/otter/json_schema/model_schemas.py#L36), I can only find the hash, not the URL :(