Fix logic bug in rev 2072

In revision 2072 "ConfigWriter: change cluster serial meaning" I misread
the serial_no update logic: it was about updating the serial number on
the object itself, not on the cluster.

So we don't actually have at all cluster serial number increase when a
node is changed (not removed/added).

This patch revers to the original always increase the target serial
number and adds increase of the cluster serial number in case a node has
been changed.

Reviewed-by: ultrotter