Fix IScalingGroup.create_webhooks

So when you create a webhook, you get the JSON blob of the webhook, which contains a link to itself as a resource (something like `http://.../groups/<group_id>/policies/<policy_id>/webhooks/<webhook_id>`).  This resource has info like the name of the webhook, some metadata, and a capability URL, which is an anonymous URL when posted to, executes the policy.  So there's that anonymous URL, and there's the URL that lets you look at and delete the resource.