[PATCH] add support for kerberos authentication

Hi all, could we consider the two following patches? We need something like that for our internal Copr instance and having it upstream would simplify a lot our downstream maintenance; it could be useful for somebody anyway. Patch tries to untie Fedora's AuthN and Copr database a little to allow different types of authentication. Only the "user" part is stored in db when applied (not whole http://user.id.fedoraproject.org/). Adds alternative (configurable) kerberos authentication. It would be nice if OpenID was generalized the same way, I'll possibly try to look at it when somebody will be interested. Paael