Classification-Changes-2.2. Option precedence for templates and their spawned classes

Implements Item 2.2 from https://gitlab.isc.org/isc-projects/kea/-/wikis/Designs//Potential-Classification-Changes

Currently precedence belongs to the template options rather than the options in classes spawned from the template. Reversing this allows the spawned classes to inherit default values yet have the ability to override them.

This is minor code change which will result in more behavior that is more useful and intuitive. It is also backward compatible in that users who need sub-class specific options cannot currently set a default via the template, rather they have to set them explicitly in every sub-class. This change won't break their current configuration.

The change is a matter of swapping the order in Pkt::addSubClass(), adding UTs, and updating documentation in the ARM.