selftest: fix envvars for creation of default user in wait_for_start

selftest: fix envvars for creation of default user in wait_for_start

Resolves failure of ad_member to start up under ad_dc (if
the user is determined to be needed).

Signed-off-by: Jamie McClymont <jamiemcclymont@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>
BUG: https://bugzilla.samba.org/show_bug.cgi?id=13225

Autobuild-User(master): Garming Sam <garming@samba.org>
Autobuild-Date(master): Tue Jan 16 07:12:01 CET 2018 on sn-devel-144

(cherry picked from commit 7a3f97f2662c6197913aeb50e5e3c0c09ff8307f)