Move keystone support from middleware to auth backend

Keystone in Horizon is currently implemented as a middleware which hacks the user object into the request. This is occasionally buggy, and causes unexpected session behavior. It also bypasses some of the expectations that Django depends on for security. Django includes a good framework for writing authentication backends, and Horizon should use these for keystone integration. This will increase the flexibility of horizon to accommodate other auth backends, and improve forward compatibility as Django evolves.