modules/aws/ami: Use implicit provider inheritance

Docs in [1], where they say:

  This approach is recommended in the common case where only a single
  configuration is needed for each provider across the entire
  configuration.

I didn't know about implicit provider inheritance when I landed the
module in b23621f (modules/aws/ami: Add a new module to get CoreOS
AMIs, 2018-07-31, openshift#84).  We'll probably drop this module soon as we
move to Red Hat CoreOS, but I worked up the change while looking for
patterns to follow for a bootstrap module.

The module source syntax used in the README is documented in [2,3,4],
and means "the modules/aws/ami subdirectory of the
github.com/openshift/installer repository cloned over HTTPS".

[1]: https://www.terraform.io/docs/modules/usage.html#implicit-provider-inheritance
[2]: https://www.terraform.io/docs/configuration/modules.html#source
[3]: https://www.terraform.io/docs/modules/sources.html#github
[4]: https://www.terraform.io/docs/modules/sources.html#modules-in-package-sub-directories

modules/aws/ami/README.md

@@ -1,27 +1,40 @@
 # Container Linux AMI Module
 
 This [Terraform][] [module][] supports `latest` versions for [Container Linux][container-linux] release channels and returns an appropriate [AMI][].
+It uses [implicit provider inheritance][implicit-provider-inheritance] to access the [AWS provider][AWS-provider].
 
 ## Example
 
-From the module directory:
+Set up a `main.tf` with:
+
+```hcl
+provider "aws" {
+  region = "us-east-1"
+}
+
+module "ami" {
+  source = "github.com/openshift/installer//modules/aws/ami"
+}
+
+output "ami" {
+  value = "${module.ami.id}"
+}
+```
+
+You can set `release_channel` and `release_version` if you need a specific Container Linux install.
+
+Then run:
 
 ```console
 $ terraform init
-$ terraform apply --var region=us-east-1
-$ terraform output id
-ami-ab6963d4
-$ terraform apply --var region=us-east-1 --var release_channel=alpha
-$ terraform output id
-ami-985953e7
-$ terraform apply --var region=us-east-2 --var release_channel=alpha --var release_version=1814.0.0
-$ terraform output id
-ami-c25f66a7
+$ terraform apply
+$ terraform output ami
+ami-00cc4337762ba4a52
 ```
 
-When you're done, clean up by removing the `.terraform` directory created by `init` and the `terraform.tfstate*` files created by `apply`.
-
 [AMI]: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/AMIs.html
+[AWS-provider]: https://www.terraform.io/docs/providers/aws/
 [container-linux]: https://coreos.com/os/docs/latest/
+[implicit-provider-inheritance]: https://www.terraform.io/docs/modules/usage.html#implicit-provider-inheritance
 [module]: https://www.terraform.io/docs/modules/
 [Terraform]: https://www.terraform.io/

modules/aws/ami/main.tf

@@ -1,8 +1,3 @@
-provider "aws" {
-  region  = "${var.region}"
-  version = "1.8.0"
-}
-
 locals {
   ami_owner = "595879546273"
   arn       = "aws"

modules/aws/ami/variables.tf

@@ -1,12 +1,3 @@
-variable "region" {
-  type = "string"
-
-  description = <<EOF
-This is the AWS region.
-It is passed through to the Terraform aws provider: https://www.terraform.io/docs/providers/aws/#region
-EOF
-}
-
 variable "release_channel" {
   type    = "string"
   default = "stable"

modules/aws/master/main.tf

@@ -5,7 +5,6 @@ locals {
 module "ami" {
   source = "../ami"
 
-  region          = "${var.region}"
   release_channel = "${var.container_linux_channel}"
   release_version = "${var.container_linux_version}"
 }

modules/aws/master/variables.tf

@@ -80,15 +80,6 @@ variable "elb_console_id" {
   type = "string"
 }
 
-variable "region" {
-  type = "string"
-
-  description = <<EOF
-This is the AWS region.
-It is passed through to the Terraform aws provider: https://www.terraform.io/docs/providers/aws/#region
-EOF
-}
-
 variable "root_volume_iops" {
   type        = "string"
   default     = "100"

modules/aws/worker/main.tf

@@ -5,7 +5,6 @@ locals {
 module "ami" {
   source = "../ami"
 
-  region          = "${var.region}"
   release_channel = "${var.container_linux_channel}"
   release_version = "${var.container_linux_version}"
 }

modules/aws/worker/variables.tf

@@ -48,15 +48,6 @@ variable "extra_tags" {
   default     = {}
 }
 
-variable "region" {
-  type = "string"
-
-  description = <<EOF
-This is the AWS region.
-It is passed through to the Terraform aws provider: https://www.terraform.io/docs/providers/aws/#region
-EOF
-}
-
 variable "root_volume_type" {
   type        = "string"
   description = "The type of volume for the root block device."

steps/bootstrap/aws/main.tf

@@ -42,7 +42,6 @@ data "ignition_config" "bootstrap_redirect" {
 module "ami" {
   source = "../../../modules/aws/ami"
 
-  region          = "${var.tectonic_aws_region}"
   release_channel = "${var.tectonic_container_linux_channel}"
   release_version = "${var.tectonic_container_linux_version}"
 }

steps/infra/aws/main.tf

@@ -42,7 +42,6 @@ module "masters" {
   master_sg_ids           = "${concat(var.tectonic_aws_master_extra_sg_ids, list(module.vpc.master_sg_id))}"
   private_endpoints       = "${local.private_endpoints}"
   public_endpoints        = "${local.public_endpoints}"
-  region                  = "${var.tectonic_aws_region}"
   root_volume_iops        = "${var.tectonic_aws_master_root_volume_iops}"
   root_volume_size        = "${var.tectonic_aws_master_root_volume_size}"
   root_volume_type        = "${var.tectonic_aws_master_root_volume_type}"
@@ -62,7 +61,6 @@ module "workers" {
   extra_tags              = "${var.tectonic_aws_extra_tags}"
   instance_count          = "${var.tectonic_worker_count}"
   load_balancers          = "${var.tectonic_aws_worker_load_balancers}"
-  region                  = "${var.tectonic_aws_region}"
   root_volume_iops        = "${var.tectonic_aws_worker_root_volume_iops}"
   root_volume_size        = "${var.tectonic_aws_worker_root_volume_size}"
   root_volume_type        = "${var.tectonic_aws_worker_root_volume_type}"