Make module idempotent by requiring subnet_id and ignore changes in several arguments (fixes #10)

examples/basic/main.tf

@@ -51,6 +51,7 @@ module "ec2" {
   name                        = "example"
   ami                         = "${data.aws_ami.amazon_linux.id}"
   instance_type               = "t2.micro"
+  subnet_id                   = "${element(data.aws_subnet_ids.all.ids, 0)}"
   vpc_security_group_ids      = ["${module.security_group.this_security_group_id}"]
   associate_public_ip_address = true
 }

main.tf

@@ -27,7 +27,6 @@ resource "aws_instance" "this" {
   source_dest_check                    = "${var.source_dest_check}"
   disable_api_termination              = "${var.disable_api_termination}"
   instance_initiated_shutdown_behavior = "${var.instance_initiated_shutdown_behavior}"
-  availability_zone                    = "${var.availability_zone}"
   placement_group                      = "${var.placement_group}"
   tenancy                              = "${var.tenancy}"
 
@@ -35,4 +34,10 @@ resource "aws_instance" "this" {
   # network_interface = "${var.network_interface}"
 
   tags = "${merge(var.tags, map("Name", format("%s-%d", var.name, count.index+1)))}"
+  lifecycle {
+    # Due to several known issues in Terraform AWS provider related to arguments of aws_instance:
+    # (eg, https://github.com/terraform-providers/terraform-provider-aws/issues/2036)
+    # we have to ignore changes in the following arguments
+    ignore_changes = ["private_ip", "vpc_security_group_ids", "root_block_device"]
+  }
 }

variables.tf

@@ -11,11 +11,6 @@ variable "ami" {
   description = "ID of AMI to use for the instance"
 }
 
-variable "availability_zone" {
-  description = "The AZ to start the instance in"
-  default     = ""
-}
-
 variable "placement_group" {
   description = "The Placement Group to start the instance in"
   default     = ""
@@ -62,7 +57,6 @@ variable "vpc_security_group_ids" {
 
 variable "subnet_id" {
   description = "The VPC Subnet ID to launch in"
-  default     = ""
 }
 
 variable "associate_public_ip_address" {