Accept existing Vignette URLs

Accept existing Vignette URLs for getThumbURL(), in order to change the
mode of an existing thumbnail or convert a full-sized image to a
thumbnail.
Yes, that would look cleaner. However, I think a getParametersFromUrl method would needlessly create more work for the class that isn't needed. We can easily grab the root portion of the existing thumbnailer URL, without needing to parse out, for example, the bucket and image path. I think function re-use and uniformity is valuable, but in this case I'm not quite convinced.