Terraform - Remove the need for region specific reference data (#1962)

* Dynamically retrieve aws_bastion_ami latest reference by querying AWS rather than hard coded

* Dynamically retrieve the list of availability_zones instead of needing to have them hard coded

* Limit availability zones to first 2, using slice extrapolation function

* Replace the need for hardcoded variable "aws_cluster_ami" by the data provided by Terraform

* Move ami choosing to vars, so people don't need to edit create infrastructure if they want another vendor image (as suggested by @atoms)

* Make name of the data block agnostic of distribution, given there are more than one distribution supported

* Add documentation about other distros being supported and what to change in which location to make these changes

contrib/terraform/aws/variables.tf

@@ -20,6 +20,21 @@ variable "aws_cluster_name" {
   description = "Name of AWS Cluster"
 }
 
+data "aws_ami" "distro" {
+  most_recent = true
+
+  filter {
+    name   = "name"
+    values = ["CoreOS-stable-*"]
+  }
+
+  filter {
+    name   = "virtualization-type"
+    values = ["hvm"]
+  }
+
+  owners = ["595879546273"] #CoreOS
+}
 
 //AWS VPC Variables
 
@@ -27,11 +42,6 @@ variable "aws_vpc_cidr_block" {
   description = "CIDR Block for VPC"
 }
 
-variable "aws_avail_zones" {
-  description = "Availability Zones Used"
-  type = "list"
-}
-
 variable "aws_cidr_subnets_private" {
   description = "CIDR Blocks for private subnets in Availability Zones"
   type = "list"
@@ -44,10 +54,6 @@ variable "aws_cidr_subnets_public" {
 
 //AWS EC2 Settings
 
-variable "aws_bastion_ami" {
-    description = "AMI ID for Bastion Host in chosen AWS Region"
-}
-
 variable "aws_bastion_size" {
     description = "EC2 Instance Size of Bastion Host"
 }
@@ -81,9 +87,6 @@ variable "aws_kube_worker_size" {
     description = "Instance size of Kubernetes Worker Nodes"
 }
 
-variable "aws_cluster_ami" {
-    description = "AMI ID for Kubernetes Cluster"
-}
 /*
 * AWS ELB Settings
 *