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/modules/kubernetes-cluster/variables.tf

@@ -39,3 +39,6 @@ variable "private_network_cidr" {
 variable "private_subnet_cidr" {
   default = "10.0.10.0/24"
 }
+variable "network_zone" {
+  default = "eu-central"
+}