cache_req: make sure that we always fetch default attrs
    
    Some plug-ins allow to specify what attributes should be returned,
    but we always need to make sure that operational attributes such
    as expiration time is returned so we can calculate the expiration
    status.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3151
    
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>