add intermediate version of BGP backbone

This adds (incomplete) support for configuring the BGP backbone that
could until now only be deployed locally using shell scripts.

The idea is to share the CSV-based config files between the shell
scripts and the new Ansible-based approach to allow peaceful coexistence
of both deployment variants. Note that the Ansible-based approach
verifies input data much stricter than the shell scripts to prevent
common user errors, so not everything will work.

For Ansible to find the config CSVs, a symlink "backbone-conf" to the
conf directory of the backbone repo[1] is expected in the ansible root
directory.

Currently, the ansible-based approach can only deploy Wireguard tunnels,
but will be extended to also configure Bird2 sessions.

[1] https://github.com/freifunk-stuttgart/backbone