CVE-2022-32744 s4:kdc: Modify HDB plugin to only look up kpasswd principal

This plugin is now only used by the kpasswd service. Thus, ensuring we
only look up the kadmin/changepw principal means we can't be fooled into
accepting tickets for other service principals. We make sure not to
specify a specific kvno, to ensure that we do not accept RODC-issued
tickets.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15074

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>
