dev-docs: add on-prem terraform to vpn setup (#2619)

* vpn: add fake-on-prem infra

* dev-docs: move vpn helm

dev-docs/howto/vpn/on-prem-terraform/variables.tf

@@ -0,0 +1,32 @@
+variable "resource_group_location" {
+  type        = string
+  default     = "westeurope"
+  description = "Location of the resource group."
+}
+
+variable "name_prefix" {
+  type    = string
+  default = "vpn-test"
+}
+
+variable "remote_addr" {
+  type        = string
+  description = "The public IP address of the remote host."
+}
+
+variable "ike_psk" {
+  type        = string
+  description = "The IKE pre-shared key."
+}
+
+variable "local_ts" {
+  type        = string
+  description = "The local traffic selector."
+  default     = "10.99.0.0/16"
+}
+
+variable "remote_ts" {
+  type        = list(string)
+  description = "The remote traffic selector."
+  default     = ["10.10.0.0/16", "10.96.0.0/12"]
+}