Make test_quotas compatible with nosetests

Nosetests detects any function that ends with `test` as a test function which
causes `is_v20_api_test` to be run as a test. This fails because `plugin`
won't be defined yet.

In the interest of being developer friendly and supporting this common
test-runner, we should prefix with an underscore so it doesn't get picked up
as an actual test.

Change-Id: I0802b9f2271cc6f555a409784cfb52911098c2d6
Merge "Make test_quotas compatible with nosetests"