Change container-linux module preference to Flatcar Linux

* No change to Fedora CoreOS modules
* For Container Linx AWS and Azure, change the `os_image` default
from coreos-stable to flatcar-stable
* For Container Linux GCP and DigitalOcean, change `os_image` to
be required since users should upload a Flatcar Linux image and
set the variable
* For Container Linux bare-metal, recommend users change the
`os_channel` to Flatcar Linux. No actual module change.

google-cloud/container-linux/kubernetes/variables.tf

@@ -48,8 +48,7 @@ variable "worker_type" {
 
 variable "os_image" {
   type        = string
-  description = "Container Linux image for compute instances (e.g. coreos-stable)"
-  default     = "coreos-stable"
+  description = "Container Linux image for compute instances (e.g. coreos-stable, custom-image)"
 }
 
 variable "disk_size" {
@@ -83,12 +82,6 @@ variable "ssh_authorized_key" {
   description = "SSH public key for user 'core'"
 }
 
-variable "asset_dir" {
-  type        = string
-  description = "Absolute path to a directory where generated assets should be placed (contains secrets)"
-  default     = ""
-}
-
 variable "networking" {
   type        = string
   description = "Choice of networking provider (flannel or calico)"
@@ -131,6 +124,12 @@ variable "worker_node_labels" {
 
 # unofficial, undocumented, unsupported
 
+variable "asset_dir" {
+  type        = string
+  description = "Absolute path to a directory where generated assets should be placed (contains secrets)"
+  default     = ""
+}
+
 variable "cluster_domain_suffix" {
   type        = string
   description = "Queries for domains with the suffix will be answered by coredns. Default is cluster.local (e.g. foo.default.svc.cluster.local) "

google-cloud/container-linux/kubernetes/workers/variables.tf

@@ -37,7 +37,6 @@ variable "machine_type" {
 variable "os_image" {
   type        = string
   description = "Container Linux image for compute instanges (e.g. gcloud compute images list)"
-  default     = "coreos-stable"
 }
 
 variable "disk_size" {