PAM: add pam_sss option allow_missing_name
    
    With this option SSSD can be used with the gdm Smartcard feature.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2941
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit 325ed9f92f1ea1f348fd7913229faecf3dc1d40b)