[useradmin] Fix LDAP sync (ldap_access.py) certificate validation logic

Even though admin handn't set ldap_cert (it was null), authentication via ldap would work, but they ldap synchronization would fail.

It seems we fixed the certificate validation logic in auth/backend.py, but we did not do so in useradmin/ldap_access.py.

The following uses an insecure option "ALLOW" which allows TLS even if a certificate is invalid.
Also, the real bug here is that it assumes that the default does not require validation.

    if cert_file is not None:
      ldap.set_option(ldap.OPT_X_TLS_REQUIRE_CERT, ldap.OPT_X_TLS_ALLOW)
      ldap.set_option(ldap.OPT_X_TLS_CACERTFILE, cert_file)