Fixes for Hetzner terraform and Hetzner Cloud (#8702)

* - add ability to specify the network_zone in hetzner terraform
- Export the network id from hetzner terraform the the generated inventory.ini

* - Add with_networks variable to allow different deployments of hcloud controller manager

- Add network id to hcloud controller secret (added via the inventory)

- Don't include extra_args if it's not set

contrib/terraform/hetzner/templates/inventory.tpl

@@ -14,3 +14,6 @@ ${list_worker}
 [k8s-cluster:children]
 kube-master
 kube-node
+
+[k8s-cluster:vars]
+network_id=${network_id}