By adding a top-level element called "_meta" to the --list response,
Ansible will not call the inventory script with --host for each host
thus saving a lot of time and many requests to the Hetzner Cloud API.

The speed-up is significant; `ansible-inventory --list` is down from
about 1 minute to just 7 seconds in my testing (with ~60ms latency).

From 2.0[0] onwards, all `ansible_ssh_*` variables are deprecated.

[0]: https://docs.ansible.com/ansible/2.3/intro_inventory.html

This has multiple benefits:
- We don't have to wait until Ansible 2.8 (which will be when the official hcloud dynamic inventory provider will be included)
- We can expand the script to group servers by labels if required
- We can use the vault to automatically get the hetzner cloud token which is not possible otherwise