feat: fetch latest focal image if not specified

ami.tf

@@ -0,0 +1,25 @@
+data "aws_ami" "default_image" {
+  most_recent = true
+  name_regex  = "^ubuntu/.*focal-20.04-amd64-server-\\d+(\\.\\d+)?$"
+  owners      = ["099720109477"]
+
+  filter {
+    name   = "name"
+    values = ["ubuntu/*focal*"]
+  }
+
+  filter {
+    name   = "architecture"
+    values = ["x86_64"]
+  }
+
+  filter {
+    name   = "root-device-type"
+    values = ["ebs"]
+  }
+
+  filter {
+    name   = "virtualization-type"
+    values = ["hvm"]
+  }
+}

k8s.tf

@@ -237,7 +237,7 @@ resource "kops_instance_group" "masters" {
   cluster_name = kops_cluster.k8s.id
   name         = "${var.control_plane_prefix}-${var.region}${each.key}"
   role         = "ControlPlane"
-  image        = var.master_image != null ? var.master_image : var.image
+  image        = coalesce(var.master_image, var.image, "${data.aws_ami.default_image.owner_id}/${data.aws_ami.default_image.name}")
   min_size     = 1
   max_size     = 1
   machine_type = var.master_types[0]
@@ -272,7 +272,7 @@ resource "kops_instance_group" "nodes" {
   cluster_name = kops_cluster.k8s.id
   name         = "nodes-${each.key}"
   role         = "Node"
-  image        = var.node_image != null ? var.node_image : var.image
+  image        = coalesce(var.node_image, var.image, "${data.aws_ami.default_image.owner_id}/${data.aws_ami.default_image.name}")
   min_size     = lookup(local.min_nodes, each.key)
   max_size     = lookup(local.max_nodes, each.key)
   machine_type = var.node_types[0]
@@ -311,7 +311,7 @@ resource "kops_instance_group" "additional_nodes" {
   cluster_name = kops_cluster.k8s.id
   name         = "nodes-${each.key}"
   role         = "Node"
-  image        = each.value.image != null ? each.value.image : var.image
+  image        = coalesce(each.value.image, var.image, "${data.aws_ami.default_image.owner_id}/${data.aws_ami.default_image.name}")
   min_size     = each.value.min_size
   max_size     = each.value.max_size
   machine_type = each.value.types[0]