Support drop-in configs

[Cornelicorn]

Allowing drop-in configs (i.e./etc/sbctl/sbct.conf.d/*.conf) would be nice for use in bigger setups, where sbctl is used on different machines with different requirements.

For example all machines should get custom db certificates, but some machines should also get Microsoft's certificates for Nvidia devices, so you could create /etc/sbctl/sbctl.conf.d/nvidia.conf containing

db_additions:
  - microsoft

on machines with an nvidia card and use the same invocations of sbctl throughout all hosts.

This would introduce more complexity to the config parsing, but also allows distros shipping their own config, which users then still can override while keeping the distros defaults valid and updated on package upgrades.

[Foxboron]

It was part of my long-term plan but I didn't want the config stuff to drag on forever so limited it to only a single file for the first iteration :)