Add an option to recreate a chroot during maintenance

[spanezz]

For chroots based on unstable, fast moving distributions, it may be less error-prone to keep them up to date by recreating them during maintenance, instead of just upgrading them.

Add a configuration option to declare that a maintenance operation will always recreate the chroot from scratch before running the playbook, even if it already exists

[spanezz]

Done in 14c5eb7.

Use nspawn_runner_maint_recreate: on in playbook vars to activate this feature