dsdb: Use samdb_get_system_container_dn() to get Password Settings Container

dsdb: Use samdb_get_system_container_dn() to get Password Settings Container

By doing this we use the common samdb_get_system_container_dn() routine and we
avoid doing a linerize and parse step on the main DN, instead using the
already stored parse of the DN.  This is more hygenic.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=9959

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
(cherry picked from commit 3669caa97f76d3e893ac6a1ab88341057929ee6a)