Add sugar for creating containing directories with specified ownership/perms

Created originally in Butane by @bgilbert: https://github.com/coreos/butane/issues/380

Ignition creates containing directories for files/dirs/links specified in the config, but it gives them default ownership/permissions rather than the ones specified for the file/dir/link. This ensures that containing directories have safe default ownership. It's still possible to override the parents' ownership/perms by explicitly specifying the containing directories in the directories section.

But it's not especially ergonomic. A deeply-nested file might need several containing directories, each of which needs to be spelled out explicitly. This makes sense for Ignition configs, which are intentionally low-level, but it'd be nice to have Butane sugar to avoid this.

The main issue here will be syntax. Butane doesn't know which directories already exist, and we can't override perms on every containing directory all the way up to the root. For example, if creating /home/builder/.config/systemd/user/default.target.wants/something.service, we might want to set ownership for builder/.config/systemd/user/default.target.wants but not /home. I don't think the ecosystem has a conventional syntax for this.

We could do something like this:

variant: fcos
version: 1.5.0-experimental
storage:
  files:
    - path: /home/builder/.config/systemd/user/default.target.wants/something.service
      user:
        name: builder
      contents:
        inline: invalid
  directories:
    - path: /home/builder/.config/systemd/user/default.target.wants
      base: /home
      user:
        name: builder

That has the disadvantages of being verbose and difficult to understand.

We could merge the "parent directories" fields into the files/directories/links sections, but we can't reuse a file mode as a directory mode, so we might want another field or so:

variant: fcos
version: 1.5.0-experimental
storage:
  files:
    - path: /home/builder/.config/systemd/user/default.target.wants/something.service
      user:
        name: builder
      contents:
        inline: invalid
      parents:
        base: /home
        mode: 0700

Discussion thread

Start by reading the linked Butane issue and the Fedora discussion thread, then inspect how Butane represents storage files, directories, and links. The issue names no files or tests; done requires an agreed syntax for parent-directory ownership and permissions plus implementation and coverage for the proposed behavior.