Simplify butane configuration

[Okeanos]

The butane configuration, in particular for the storage.file is unnecessarily verbose.

There are some already existing ways to simplify this such as storage.trees.

Further optimisation options theoretically exist, such as shared common configs, that can be merged at transpile time (see ignition.config.merge. However, issues coreos-docs#163, butane#118, and butane#275 make this impractical.