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