Configure Cleaning

I just discovered recently that over a month of scheduled backups that should have been occurring do not exist. This was because I needed to run `btrbk clean` to remove stray subvolumes.

In order to prevent this from happening again, I suppose I could make a cronjob that runs `btrbk clean` hourly.. But I'd prefer to manage everything within btrbk. Is there a flag or option I can set in the configuration that will automatically clean out stray subvolumes?

Start by reading the existing `btrbk clean` command and configuration documentation to understand how stray subvolumes are handled. Clarify the intended configuration and scheduling behavior, then identify the relevant implementation and tests; done means an agreed configuration path performs the requested cleanup without a cron job.

Written by the indexing model from the issue text.