Workaround to allow use of ed25519 keys on Azure

* Allow passing a dummy RSA key to Azure to satisfy its obtuse
requirements (recommend deleting the corresponding private key)
* Then `ssh_authorized_key` can be used to provide Fedora CoreOS
or Flatcar Linux with a modern ed25519 public key to set in the
authorized_keys via Ignition

azure/flatcar-linux/kubernetes/workers/variables.tf

@@ -79,6 +79,12 @@ variable "ssh_authorized_key" {
   description = "SSH public key for user 'core'"
 }
 
+variable "azure_authorized_key" {
+  type        = string
+  description = "Optionally, pass a dummy RSA key to satisfy Azure validations (then use an ed25519 key set above)"
+  default     = ""
+}
+
 variable "service_cidr" {
   type        = string
   description = <<EOD

azure/flatcar-linux/kubernetes/workers/workers.tf

@@ -3,6 +3,8 @@ locals {
   channel      = split("-", var.os_image)[1]
   offer_suffix = var.arch == "arm64" ? "corevm" : "free"
   urn          = var.arch == "arm64" ? local.channel : "${local.channel}-gen2"
+
+  azure_authorized_key = var.azure_authorized_key == "" ? var.ssh_authorized_key : var.azure_authorized_key
 }
 
 # Workers scale set
@@ -48,7 +50,7 @@ resource "azurerm_linux_virtual_machine_scale_set" "workers" {
   admin_username = "core"
   admin_ssh_key {
     username   = "core"
-    public_key = var.ssh_authorized_key
+    public_key = local.azure_authorized_key
   }
 
   # network