selftest/Samba3: make --log-stdout optional

selftest/Samba3: make --log-stdout optional

It's still used by default, but the following environment variables
are able to avoid it NMBD_DONT_LOG_STDOUT=1, WINBINDD_DONT_LOG_STDOUT=1
and SMBD_DONT_LOG_STDOUT=1.

It's strange that nmbd and smbd take --log-stdout, while winbindd
takes --stdout.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Sat Feb  1 01:27:18 CET 2014 on sn-devel-104