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/variables.tf

@@ -1,6 +1,10 @@
 variable "zone" {
   description = "The zone where to run the cluster"
 }
+variable "network_zone" {
+  description = "The network zone where the cluster is running"
+  default = "eu-central"
+}
 
 variable "prefix" {
   description = "Prefix for resource names"