Rename container-linux modules to flatcar-linux

* CoreOS Container Linux was deprecated in v1.18.3
* Continue transitioning docs and modules from supporting
both CoreOS and Flatcar "variants" of Container Linux to
now supporting Flatcar Linux and equivalents

Action Required: Update the Flatcar Linux modules `source`
to replace `s/container-linux/flatcar-linux`. See docs for
examples

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

@@ -0,0 +1,99 @@
+variable "name" {
+  type        = string
+  description = "Unique name for the worker pool"
+}
+
+# Azure
+
+variable "region" {
+  type        = string
+  description = "Must be set to the Azure Region of cluster"
+}
+
+variable "resource_group_name" {
+  type        = string
+  description = "Must be set to the resource group name of cluster"
+}
+
+variable "subnet_id" {
+  type        = string
+  description = "Must be set to the `worker_subnet_id` output by cluster"
+}
+
+variable "security_group_id" {
+  type        = string
+  description = "Must be set to the `worker_security_group_id` output by cluster"
+}
+
+variable "backend_address_pool_id" {
+  type        = string
+  description = "Must be set to the `worker_backend_address_pool_id` output by cluster"
+}
+
+# instances
+
+variable "worker_count" {
+  type        = number
+  description = "Number of instances"
+  default     = 1
+}
+
+variable "vm_type" {
+  type        = string
+  description = "Machine type for instances (see `az vm list-skus --location centralus`)"
+  default     = "Standard_DS1_v2"
+}
+
+variable "os_image" {
+  type        = string
+  description = "Channel for a Container Linux derivative (flatcar-stable, flatcar-beta, flatcar-alpha, flatcar-edge)"
+  default     = "flatcar-stable"
+}
+
+variable "priority" {
+  type        = string
+  description = "Set priority to Spot to use reduced cost surplus capacity, with the tradeoff that instances can be evicted at any time."
+  default     = "Regular"
+}
+
+variable "snippets" {
+  type        = list(string)
+  description = "Container Linux Config snippets"
+  default     = []
+}
+
+# configuration
+
+variable "kubeconfig" {
+  type        = string
+  description = "Must be set to `kubeconfig` output by cluster"
+}
+
+variable "ssh_authorized_key" {
+  type        = string
+  description = "SSH public key for user 'core'"
+}
+
+variable "service_cidr" {
+  type        = string
+  description = <<EOD
+CIDR IPv4 range to assign Kubernetes services.
+The 1st IP will be reserved for kube_apiserver, the 10th IP will be reserved for coredns.
+EOD
+  default     = "10.3.0.0/16"
+}
+
+variable "node_labels" {
+  type        = list(string)
+  description = "List of initial node labels"
+  default     = []
+}
+
+# unofficial, undocumented, unsupported
+
+variable "cluster_domain_suffix" {
+  description = "Queries for domains with the suffix will be answered by coredns. Default is cluster.local (e.g. foo.default.svc.cluster.local) "
+  type        = string
+  default     = "cluster.local"
+}
+