modules/aws/etcd: Add a README

READMEs are part of the standard module structure [1], and docs like
this should make onboarding new contributors easier.  The example's
etcd ports are from [2,3].  The egress rule is based on [4], although
I've restricted it to the VPC.

I've also added some defaults, so users who are not interested
lower-level settings can ignore them.  You can find Container Linux
AMIs in [5] if you want to test ec2_ami.  I've added a dependency on
modules/container_linux to get local container_linux_version support
for 'lastest'.  Without that, even users setting ec2_ami would need to
set a major.minor.patch container_linux_version to avoid hitting:

  * module.etcd.data.aws_ami.coreos_ami: data.aws_ami.coreos_ami: Your
    query returned no results. Please change your search criteria and
    try again.

Some of the module variables are just passed through to lower-level
providers (e.g. ssh_key is passed through to aws_instance's key_name).
That sort of thing is awkward at the moment, because there's no
generic way to declare a variable unset (vs. setting it to some sort
of zero value).  Terraform will grow support for that distinction (via
a 'null' value) in 0.12 as part of HCL2 [6,7].  Terraform 0.12 is due
out in the next few months [7].

The hcl pretty printing is supported by Linguist [8], which GitHub
uses for syntax highlighting [9].

[1]: https://www.terraform.io/docs/modules/create.html#standard-module-structure
[2]: https://github.com/coreos/etcd/blob/master/README.md#etcd-tcp-ports
[3]: http://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.txt
[4]: https://www.terraform.io/docs/providers/aws/r/security_group.html#argument-reference
[5]: https://coreos.com/os/docs/latest/booting-on-ec2.html
[6]: hashicorp/terraform#5471 (comment)
[7]: https://www.hashicorp.com/blog/terraform-0-1-2-preview
[8]: https://github.com/github/linguist/blob/v6.3.1/lib/linguist/languages.yml#L1723-L1733
[9]: https://help.github.com/articles/creating-and-highlighting-code-blocks/#syntax-highlighting

modules/aws/etcd/README.md

@@ -0,0 +1,68 @@
+# AWS Etcd Module
+
+This [Terraform][] [module][] makes it easy to create [etcd][] nodes on [AWS][].
+
+Read the [etcd recommended hardware guide][hardware] for best performance.
+
+## Example
+
+```hcl
+provider "aws" {
+  region = "us-east-1"
+}
+
+resource "aws_s3_bucket" "etcd_ignition" {
+}
+
+resource "aws_vpc" "example" {
+  cidr_block           = "10.0.0.0/16"
+  enable_dns_hostnames = true
+  enable_dns_support   = true
+}
+
+resource "aws_subnet" "example" {
+  vpc_id     = "${aws_vpc.example.id}"
+  cidr_block = "${aws_vpc.example.cidr_block}"
+}
+
+resource "aws_security_group" "etcd" {
+  vpc_id = "${aws_vpc.example.id}"
+
+  ingress {
+    from_port   = 2379
+    to_port     = 2380
+    protocol    = "tcp"
+    cidr_blocks = ["${aws_subnet.example.cidr_block}"]
+  }
+
+  egress {
+    from_port   = 0
+    to_port     = 0
+    protocol    = "-1"
+    cidr_blocks = ["${aws_subnet.example.cidr_block}"]
+  }
+}
+
+module "etcd" {
+  source = "github.com/openshift/installer//modules/aws/etcd"
+
+  base_domain    = "openshift.example.com"
+  cluster_id     = "123"
+  cluster_name   = "my-cluster"
+  instance_count = "3"
+  s3_bucket      = "${aws_s3_bucket.etcd_ignition.id}"
+  sg_ids         = ["${aws_security_group.etcd.id}"]
+  subnets        = ["${aws_subnet.example.id}"]
+}
+```
+
+You can set `container_linux_channel` and `container_linux_version` if you need a specific [Container Linux][container-linux] install.
+Alternatively, you can set `ec2_ami` directly if you want to use an [AMI][] that is not Container Linux.
+
+[AMI]: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/AMIs.html
+[AWS]: https://aws.amazon.com/
+[container-linux]: https://coreos.com/os/docs/latest/
+[etcd]: https://github.com/coreos/etcd
+[hardware]: https://github.com/coreos/etcd/blob/v3.3.8/Documentation/op-guide/hardware.md#example-hardware-configurations
+[module]: https://www.terraform.io/docs/modules/
+[Terraform]: https://www.terraform.io/

modules/aws/etcd/nodes.tf

@@ -3,10 +3,17 @@ locals {
   arn       = "aws"
 }
 
+module "container_linux" {
+  source = "../../container_linux"
+
+  release_channel = "${var.container_linux_channel}"
+  release_version = "${var.container_linux_version}"
+}
+
 data "aws_ami" "coreos_ami" {
   filter {
     name   = "name"
-    values = ["CoreOS-${var.container_linux_channel}-${var.container_linux_version}-*"]
+    values = ["CoreOS-${var.container_linux_channel}-${module.container_linux.version}-*"]
   }
 
   filter {

modules/aws/etcd/variables.tf

@@ -11,31 +11,36 @@ variable "cluster_name" {
 }
 
 variable "container_linux_channel" {
-  type = "string"
+  type    = "string"
+  default = "stable"
 }
 
 variable "container_linux_version" {
-  type = "string"
+  type    = "string"
+  default = "latest"
 }
 
 variable "instance_count" {
   default = "3"
 }
 
 variable "ssh_key" {
-  type = "string"
+  type    = "string"
+  default = ""
 }
 
 variable "subnets" {
   type = "list"
 }
 
 variable "container_image" {
-  type = "string"
+  type    = "string"
+  default = "quay.io/coreos/etcd:v3.3.8"
 }
 
 variable "ec2_type" {
-  type = "string"
+  type    = "string"
+  default = "t2.medium"
 }
 
 variable "ec2_ami" {
@@ -51,11 +56,13 @@ variable "extra_tags" {
 
 variable "root_volume_type" {
   type        = "string"
+  default     = "gp2"
   description = "The type of volume for the root block device."
 }
 
 variable "root_volume_size" {
   type        = "string"
+  default     = "30"
   description = "The size of the volume in gigabytes for the root block device."
 }