cache_req: allow to return well known object as result
    
    When we are looking name by sid or sid by name we want to be also
    able to translate well known sid/name into its pair equivalent.
    
    This adds the ability to mock cache_req_result if a well known object
    is found.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3151
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>