sbus: allow access for sssd user
    
    D-Bus allows access for root and euid by default, however when running
    in non-root mode monitor continues to run as root but responsers as sssd
    user. Therefore monitor euid != sssd user and the connection is terminated.
    
    We must explicitly allow the connection for sssd user uid.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3871
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>