Config file management

This is a brainstorm of sorts.

We, as a distribution, are facing a difficulty with the kea config files.

The config file for each daemon contains configuration details that are likely to remain static, like log file location, but also settings that WILL HAVE to be changed locally, like the network interface to listen on, address pools, database credentials, and so on.

This combination makes it very hard for us to ship a new configuration file, because that will cause conf file prompts.

Furthermore, many of these configuration settings are duplicated between the configuration files for each daemon, like control socket path, which means that if a user accepted a conf file prompt in one place, but not the other, the whole setup will not work anymore. See [1].

I think we should leverage the include mechanism that these config files support and move out the settings that are more likely to be changed by a user, such as the networking and address pool details, and leave in the main config the ones that we as a distribution have a stronger opinion on, like location of control sockets and logging.

This must be done very carefully, though, as these config files are json files, with rules about brackets and commas and comments.

1. https://bugs.launchpad.net/ubuntu/+source/isc-kea/+bug/1863100/comments/2