make sure outputs are always valid (#29)

Outputs directly referencing resource IDs that are only conditionally
created are not valid. Use the concat * trick to make sure we always
have a valid value for these outputs.

outputs.tf

@@ -52,7 +52,7 @@ output "database_subnets_cidr_blocks" {
 
 output "database_subnet_group" {
   description = "ID of database subnet group"
-  value       = "${aws_db_subnet_group.database.id}"
+  value       = "${element(concat(aws_db_subnet_group.database.*.id, list("")), 0)}"
 }
 
 output "elasticache_subnets" {
@@ -67,7 +67,7 @@ output "elasticache_subnets_cidr_blocks" {
 
 output "elasticache_subnet_group" {
   description = "ID of elasticache subnet group"
-  value       = "${aws_elasticache_subnet_group.elasticache.id}"
+  value       = "${element(concat(aws_elasticache_subnet_group.elasticache.*.id, list("")), 0)}"
 }
 
 # Route tables
@@ -99,22 +99,22 @@ output "natgw_ids" {
 # Internet Gateway
 output "igw_id" {
   description = "The ID of the Internet Gateway"
-  value       = "${aws_internet_gateway.this.id}"
+  value       = "${element(concat(aws_internet_gateway.this.*.id, list("")), 0)}"
 }
 
 # VPC Endpoints
 output "vpc_endpoint_s3_id" {
   description = "The ID of VPC endpoint for S3"
-  value       = "${aws_vpc_endpoint.s3.id}"
+  value       = "${element(concat(aws_vpc_endpoint.s3.*.id, list("")), 0)}"
 }
 
 output "vpc_endpoint_dynamodb_id" {
   description = "The ID of VPC endpoint for DynamoDB"
-  value       = "${aws_vpc_endpoint.dynamodb.id}"
+  value       = "${element(concat(aws_vpc_endpoint.dynamodb.*.id, list("")), 0)}"
 }
 
 # VPN Gateway
 output "vgw_id" {
   description = "The ID of the VPN Gateway"
-  value       = "${aws_vpn_gateway.this.id}"
+  value       = "${element(concat(aws_vpn_gateway.this.*.id, list("")), 0)}"
 }