config: Fix user_attributes
    
    Fixes:
    https://fedorahosted.org/sssd/ticket/3068
    
    Option user_attributes is also available in
    NSS responder, but not in PAC responder.
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    (cherry picked from commit 0a172552ec16f3b84d127399551cad786da8fd9d)