terraform-aws: variable driven ami selection (kubernetes-sigs#10520)

* modify variables.tf to accept AMI attributes via variables

* update README to guide users on utilizing variable-driven AMI configuration

* fix markdown lint error

contrib/terraform/aws/README.md

@@ -50,70 +50,32 @@ Example (this one assumes you are using Ubuntu)
 ansible-playbook -i ./inventory/hosts ./cluster.yml -e ansible_user=ubuntu -b --become-user=root --flush-cache
 ```
 
-***Using other distrib than Ubuntu***
-If you want to use another distribution than Ubuntu 18.04 (Bionic) LTS, you can modify the search filters of the 'data "aws_ami" "distro"' in variables.tf.
+## Using other distrib than Ubuntu***
 
-For example, to use:
+To leverage a Linux distribution other than Ubuntu 18.04 (Bionic) LTS for your Terraform configurations, you can adjust the AMI search filters within the 'data "aws_ami" "distro"' block by utilizing variables in your `terraform.tfvars` file. This approach ensures a flexible configuration that adapts to various Linux distributions without directly modifying the core Terraform files.
 
-- Debian Jessie, replace 'data "aws_ami" "distro"' in variables.tf with
+### Example Usages
 
-```ini
-data "aws_ami" "distro" {
-  most_recent = true
+- **Debian Jessie**: To configure the usage of Debian Jessie, insert the subsequent lines into your `terraform.tfvars`:
 
-  filter {
-    name   = "name"
-    values = ["debian-jessie-amd64-hvm-*"]
-  }
+  ```hcl
+  ami_name_pattern        = "debian-jessie-amd64-hvm-*"
+  ami_owners              = ["379101102735"]
+  ```
 
-  filter {
-    name   = "virtualization-type"
-    values = ["hvm"]
-  }
+- **Ubuntu 16.04**: To utilize Ubuntu 16.04 instead, apply the following configuration in your `terraform.tfvars`:
 
-  owners = ["379101102735"]
-}
-```
-
-- Ubuntu 16.04, replace 'data "aws_ami" "distro"' in variables.tf with
-
-```ini
-data "aws_ami" "distro" {
-  most_recent = true
+  ```hcl
+  ami_name_pattern        = "ubuntu/images/hvm-ssd/ubuntu-xenial-16.04-amd64-*"
+  ami_owners              = ["099720109477"]
+  ```
 
-  filter {
-    name   = "name"
-    values = ["ubuntu/images/hvm-ssd/ubuntu-xenial-16.04-amd64-*"]
-  }
-
-  filter {
-    name   = "virtualization-type"
-    values = ["hvm"]
-  }
-
-  owners = ["099720109477"]
-}
-```
+- **Centos 7**: For employing Centos 7, incorporate these lines into your `terraform.tfvars`:
 
-- Centos 7, replace 'data "aws_ami" "distro"' in variables.tf with
-
-```ini
-data "aws_ami" "distro" {
-  most_recent = true
-
-  filter {
-    name   = "name"
-    values = ["dcos-centos7-*"]
-  }
-
-  filter {
-    name   = "virtualization-type"
-    values = ["hvm"]
-  }
-
-  owners = ["688023202711"]
-}
-```
+  ```hcl
+  ami_name_pattern        = "dcos-centos7-*"
+  ami_owners              = ["688023202711"]
+  ```
 
 ## Connecting to Kubernetes
 

contrib/terraform/aws/variables.tf

@@ -20,20 +20,38 @@ variable "aws_cluster_name" {
   description = "Name of AWS Cluster"
 }
 
+variable "ami_name_pattern" {
+  description = "The name pattern to use for AMI lookup"
+  type        = string
+  default     = "debian-10-amd64-*"
+}
+
+variable "ami_virtualization_type" {
+  description = "The virtualization type to use for AMI lookup"
+  type        = string
+  default     = "hvm"
+}
+
+variable "ami_owners" {
+  description = "The owners to use for AMI lookup"
+  type        = list(string)
+  default     = ["136693071363"]
+}
+
 data "aws_ami" "distro" {
   most_recent = true
 
   filter {
     name   = "name"
-    values = ["debian-10-amd64-*"]
+    values = [var.ami_name_pattern]
   }
 
   filter {
     name   = "virtualization-type"
-    values = ["hvm"]
+    values = [var.ami_virtualization_type]
   }
 
-  owners = ["136693071363"] # Debian-10
+  owners = var.ami_owners
 }
 
 //AWS VPC Variables