Fix route_prefix + optimize request.context

Explations from commit messages:

Fix pyramid Configurator route_prefix usage

In case the service run under a route_prefix,
the request.matched_route.pattern contains that prefix,
so, cornice didn't apply filters because it didn't
find the route.

Reuse the request.context

Cornice service accept a "factory" parameter and pass it to the view.
The factory build an object in the pyramid framework, so, cornice
did not need to build it again.
Furthermore, by this change, factory can now be a string exactly
like in pyramid.