CVE-2018-16857 dsdb/util: Add better default lockOutObservationWindow

CVE-2018-16857 dsdb/util: Add better default lockOutObservationWindow

Clearly the lockOutObservationWindow value is important, and using a
default value of zero doesn't work very well.

This patch adds a better default value (the domain default setting of 30
minutes).

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13683

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Autobuild-User(master): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(master): Wed Nov 28 11:31:14 CET 2018 on sn-devel-144