refs https://github.com/TryGhost/Toolbox/issues/214
- The `defaultSettings` path name in the config (one pointing to routes yaml file) creates confusion with the `defaultSettings` which populate defaults for in the database settings table.
- Furthermore, the name collision creates a problem when trying to make database default settings dynamic - being able to load them from configurable file path.
- Rename makes "routing" explicit to avoid ambiguity and free up the name for the database defaults
- The value seems to be safe to be renamed as all keys used in `overrides.json` are taking priority - the name "defaultRouteSettings" hasn't surfaced at any point in the git history