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/variables.tf

@@ -0,0 +1,138 @@
+variable "cluster_name" {
+  type        = string
+  description = "Unique cluster name (prepended to dns_zone)"
+}
+
+# Azure
+
+variable "region" {
+  type        = string
+  description = "Azure Region (e.g. centralus , see `az account list-locations --output table`)"
+}
+
+variable "dns_zone" {
+  type        = string
+  description = "Azure DNS Zone (e.g. azure.example.com)"
+}
+
+variable "dns_zone_group" {
+  type        = string
+  description = "Resource group where the Azure DNS Zone resides (e.g. global)"
+}
+
+# instances
+
+variable "controller_count" {
+  type        = number
+  description = "Number of controllers (i.e. masters)"
+  default     = 1
+}
+
+variable "worker_count" {
+  type        = number
+  description = "Number of workers"
+  default     = 1
+}
+
+variable "controller_type" {
+  type        = string
+  description = "Machine type for controllers (see `az vm list-skus --location centralus`)"
+  default     = "Standard_B2s"
+}
+
+variable "worker_type" {
+  type        = string
+  description = "Machine type for workers (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 "disk_size" {
+  type        = number
+  description = "Size of the disk in GB"
+  default     = 40
+}
+
+variable "worker_priority" {
+  type        = string
+  description = "Set worker priority to Spot to use reduced cost surplus capacity, with the tradeoff that instances can be deallocated at any time."
+  default     = "Regular"
+}
+
+variable "controller_snippets" {
+  type        = list(string)
+  description = "Controller Container Linux Config snippets"
+  default     = []
+}
+
+variable "worker_snippets" {
+  type        = list(string)
+  description = "Worker Container Linux Config snippets"
+  default     = []
+}
+
+# configuration
+
+variable "ssh_authorized_key" {
+  type        = string
+  description = "SSH public key for user 'core'"
+}
+
+variable "networking" {
+  type        = string
+  description = "Choice of networking provider (flannel or calico)"
+  default     = "calico"
+}
+
+variable "host_cidr" {
+  type        = string
+  description = "CIDR IPv4 range to assign to instances"
+  default     = "10.0.0.0/16"
+}
+
+variable "pod_cidr" {
+  type        = string
+  description = "CIDR IPv4 range to assign Kubernetes pods"
+  default     = "10.2.0.0/16"
+}
+
+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 "enable_reporting" {
+  type        = bool
+  description = "Enable usage or analytics reporting to upstreams (Calico)"
+  default     = false
+}
+
+variable "enable_aggregation" {
+  type        = bool
+  description = "Enable the Kubernetes Aggregation Layer (defaults to false)"
+  default     = false
+}
+
+variable "worker_node_labels" {
+  type        = list(string)
+  description = "List of initial worker node labels"
+  default     = []
+}
+
+# unofficial, undocumented, unsupported
+
+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) "
+  default     = "cluster.local"
+}
+