YAML which is our primary user facing format has been using in-lining of
the server configuration within the preseed. But the Go and JSON side of
the struct were not set up to match, leading to a bunch of issues with
IncusOS and associated projects that frequently flip back and forth
between JSON and YAML.

To fix this properly, change the struct to reflect the way it's been
used by users since its introduction. This is technically a small
breaking change for anyone directly interacting with the struct through Go.

This isn't an actual REST API struct, so there is no impact at the REST
API level.

Signed-off-by: Stéphane Graber <[email protected]>