hcloud_inventory: Optimize --list to avoid --host calls

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).