Adjust the realm sent back based on the auth configured
    
    In an ideal world git would show the realm when asking for
    username and password, so in case the ideal world becomes
    reality, let's be nice and adjust the realm based on what
    we expect to be sent based on the authentication backend
    configured for this pagure instance.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>