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)