Basic HA+MT Configuration Template

Basic HA+MT template:

In continuation of the process of providing templates for different Kea
configurations, it would be handy if there were a template for two High
Availability (HA) Kea servers with Multi-threading enabled and employing the
dedicated listener.

The provision of a diagram would help show the difference
between HA+MT and no multithreaded HA and perhaps void the need for a new KB
article. The template configuration could also be added to "doc/examples"