Fix naming of OtterWebhooks instance variable.

scaling_group_id changed to group_id.
Fix routes in Otter, OtterConfigs, and OtterGroups

Some of the routes were not correctly migrated into Klein apps.
These routes still contained prefixes such as '/policies' that are
handled by the parent route.

'/groups' was moved further down because it is more general than other
routes under groups.

Trailing slashes were added to all leaf routes.
In response testing, check for correct code first.

Tests will error incorrectly when a route is missing, over a route not
having a transaction-id set. This moves the check for status code first,
before anything else, and clarifies the problem.
Include request URI in test for status code.
Swich mock_app to property on RequestTestMixin.
Fix tests for transaction-id on Otter app.
Default RequestTestMixin root to self.root.

This simplifies a lot of the setup and fixes tons of tests.
Remove mock_app from RequestTestMixin.

This was only being used by two tests in test_application, and setting
self.root works much better.

Changed those two tests to explicitly create a new Otter reference.