s3/winbindd: using default domain with user@domain.com format fails

s3/winbindd: using default domain with user@domain.com format fails

For example for samba client joined to a windows AD DC the following
commands fail if 'winbind use default domain = yes'
   getent passwd user@domain.com
   ssh -o user=user@domain.com localhost

The same commands succeed if the setting above has the default 'no' value

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12298

Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Mon Oct  3 23:37:44 CEST 2016 on sn-devel-144