Backups: adopting additional files backup/restore

This takes care of all configuration and state files that should be routinely backuped for Heptapod.

Rather than painfully maintaining correspondence in the Rails application by spawning a tar executable, it is much better to use the various constants we have around in py-heptapod, and it is a much easier setup for comprehensive testing than in the Rails app. We also have functional tests to confirm at the opposite of the spectrum.

Provides all what is needed for heptapod#439 (closed): we only need the Rails app to call the new commands.