Make use of vulcand dynamic bucket config

Alex it is more complex than that. If variable is provided then we for every request we just try to use it to retrieve rates. If retrieval fails for whatever reason (there is no such header in the request; the value of the header is invalid, etc.) then we use the default rate provided in the **request**, **period** and **burst**.

The property to fallback to static configuration given via  the **request**, **period** and **burst** when retrieval of rates via the configVar fails seems to me important. It allows us keep processing requests at reasonable rate when service providing rates per request fails.