db_subnet_group: fix failing output expressions (#22)

Accessing aws_db_subnet_group.this when its count = 0 is an error, so we
need to instead access it via splat syntax and then apply a default value
for when the resulting list is empty.

modules/db_subnet_group/outputs.tf

 # DB subnet group
 output "this_db_subnet_group_id" {
   description = "The db subnet group name"
-  value       = "${var.count == 0 ? "" : aws_db_subnet_group.this.id}"
+  value       = "${element(concat(aws_db_subnet_group.this.*.id, list("")), 0)}"
 }
 
 output "this_db_subnet_group_arn" {
   description = "The ARN of the db subnet group"
-  value       = "${aws_db_subnet_group.this.arn}"
+  value       = "${element(concat(aws_db_subnet_group.this.*.arn, list("")), 0)}"
 }