Make the response status code configurable

[mecclesgoogle]

https://github.com/apigee-127/volos/blob/e70f25eb04c5a1c1f63ecafa48f2e77484b5cbec/quota/common/lib/quota-connect.js#L120

Some users might want to return a HTTP 429 status code, which is technically the correct status code to return for quota violations.

To preserve backwards compatibility, it can be made configurable and defaulted to 403, giving the consumer of the library (e.g. the Apgee microgateway) the ability to override it to 429.