Rename DigitalOcean image variable to os_image

* Rename variable `image` to `os_image` to match the naming
used for the same purpose on other supported platforms (e.g.
AWS, Azure, GCP)

digital-ocean/container-linux/kubernetes/controllers.tf

@@ -1,6 +1,6 @@
 locals {
   official_images = ["coreos-stable", "coreos-beta", "coreos-alpha"]
-  is_official_image = contains(local.official_images, var.image)
+  is_official_image = contains(local.official_images, var.os_image)
 }
 
 # Controller Instance DNS records
@@ -42,7 +42,7 @@ resource "digitalocean_droplet" "controllers" {
   name   = "${var.cluster_name}-controller-${count.index}"
   region = var.region
 
-  image = var.image
+  image = var.os_image
   size  = var.controller_type
 
   # network

digital-ocean/container-linux/kubernetes/variables.tf

@@ -41,7 +41,7 @@ variable "worker_type" {
   default     = "s-1vcpu-2gb"
 }
 
-variable "image" {
+variable "os_image" {
   type        = string
   description = "Container Linux image for instances (e.g. coreos-stable)"
   default     = "coreos-stable"

digital-ocean/container-linux/kubernetes/workers.tf

@@ -31,7 +31,7 @@ resource "digitalocean_droplet" "workers" {
   name   = "${var.cluster_name}-worker-${count.index}"
   region = var.region
 
-  image = var.image
+  image = var.os_image
   size  = var.worker_type
 
   # network