Revert Flatcar Linux Azure to manual upload images

* Initial support for Flatcar Linux on Azure used the Flatcar
Linux Azure Marketplace images (e.g. `flatcar-stable`) in
poseidon/typhoon#664
* Flatcar Linux Azure Marketplace images have some unresolved
items poseidon/typhoon#703
* Until the Marketplace items are resolved, revert to requiring
Flatcar Linux's images be manually uploaded (like GCP and
DigitalOcean)

controllers.tf

@@ -53,23 +53,18 @@ resource "azurerm_linux_virtual_machine" "controllers" {
     storage_account_type = "Premium_LRS"
   }
 
-  source_image_reference {
-    publisher = local.flavor == "flatcar" ? "Kinvolk" : "CoreOS"
-    offer     = local.flavor == "flatcar" ? "flatcar-container-linux" : "CoreOS"
-    sku       = local.channel
-    version   = "latest"
-  }
-
-  # Gross hack just for Flatcar Linux
-  dynamic "plan" {
-    for_each = local.flavor == "flatcar" ? [1] : []
+  // CoreOS Container Linux or Flatcar Container Linux (manual upload)
+  dynamic "source_image_reference" {
+    for_each = local.flavor == "coreos" ? [1] : []
 
     content {
-      name      = local.channel
-      publisher = "kinvolk"
-      product   = "flatcar-container-linux"
+      publisher = "CoreOS"
+      offer     = "CoreOS"
+      sku       = local.channel
+      version   = "latest"
     }
   }
+  source_image_id = local.flavor == "coreos" ? null : var.os_image
 
   # network
   network_interface_ids = [

variables.tf

@@ -48,8 +48,7 @@ variable "worker_type" {
 
 variable "os_image" {
   type        = string
-  description = "Channel for a Container Linux derivative (coreos-stable, coreos-beta, coreos-alpha, flatcar-stable, flatcar-beta)"
-  default     = "flatcar-stable"
+  description = "Channel for a Container Linux derivative (/subscriptions/some-flatcar-upload, coreos-stable, coreos-beta, coreos-alpha)"
 }
 
 variable "disk_size" {

workers/workers.tf

@@ -24,23 +24,18 @@ resource "azurerm_linux_virtual_machine_scale_set" "workers" {
     caching              = "ReadWrite"
   }
 
-  source_image_reference {
-    publisher = local.flavor == "flatcar" ? "Kinvolk" : "CoreOS"
-    offer     = local.flavor == "flatcar" ? "flatcar-container-linux" : "CoreOS"
-    sku       = local.channel
-    version   = "latest"
-  }
-
-  # Gross hack just for Flatcar Linux
-  dynamic "plan" {
-    for_each = local.flavor == "flatcar" ? [1] : []
+  // CoreOS Container Linux or Flatcar Container Linux (manual upload)
+  dynamic "source_image_reference" {
+    for_each = local.flavor == "coreos" ? [1] : []
 
     content {
-      name      = local.channel
-      publisher = "kinvolk"
-      product   = "flatcar-container-linux"
+      publisher = "CoreOS"
+      offer     = "CoreOS"
+      sku       = local.channel
+      version   = "latest"
     }
   }
+  source_image_id = local.flavor == "coreos" ? null : var.os_image
 
   # Azure requires setting admin_ssh_key, though Ignition custom_data handles it too
   admin_username = "core"