auth: keytab invalidation fix

auth: keytab invalidation fix

chgtdcpass should add a new DC password and delete the old ones but the bug
exposed by this test causes the tool to remove only a single record from
the old entries, leaving the old passwords functional.  Since the tool is
used by administrators who may have disclosed their domain join password and
want to invalidate it, this is a security concern.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13415

Signed-off-by: Aaron Haslett <aaronhaslett@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Tue May 15 15:45:08 CEST 2018 on sn-devel-144