cleanup kea dhcpv6 config generation

[sarcasticadmin]

Description

Following the work around in #1085 we would like to make the implementation of this logic clearer.

Ideas have been to move this to jinja for templating

AC

• work around moves to a jinja template
• nixos core tests still pass

[owendelong]

Now that the bridges are not named virbr0, but bridge103 and bridge503, respectively, is this still an issue? Couldn't we (instead) implement KEA HA and have both 103 and 503 on both servers, making 103 primary on core-expo and 503 primary on core-conf?

I think this would be a better approach than that in #1085.