Enable db_subnet_group_name variable (terraform-aws-modules#38)

* ✨ Enable db_subnet_group_name variable

If the variable not specify then use the db_subnet_group module

* Disable create_db_subnet_group if db_subnet_group_name is defined

main.tf

@@ -1,10 +1,15 @@
+locals {
+  db_subnet_group_name          = "${coalesce(var.db_subnet_group_name, module.db_subnet_group.this_db_subnet_group_id)}"
+  enable_create_db_subnet_group = "${var.db_subnet_group_name == "" ? var.create_db_subnet_group : 0}"
+}
+
 ##################
 # DB subnet group
 ##################
 module "db_subnet_group" {
   source = "./modules/db_subnet_group"
 
-  count       = "${var.create_db_subnet_group}"
+  count       = "${local.enable_create_db_subnet_group}"
   identifier  = "${var.identifier}"
   name_prefix = "${var.identifier}-"
   subnet_ids  = ["${var.subnet_ids}"]
@@ -56,7 +61,7 @@ module "db_instance" {
   snapshot_identifier = "${var.snapshot_identifier}"
 
   vpc_security_group_ids = ["${var.vpc_security_group_ids}"]
-  db_subnet_group_name   = "${module.db_subnet_group.this_db_subnet_group_id}"
+  db_subnet_group_name   = "${local.db_subnet_group_name}"
   parameter_group_name   = "${module.db_parameter_group.this_db_parameter_group_id}"
 
   multi_az            = "${var.multi_az}"

variables.tf

@@ -80,11 +80,11 @@ variable "vpc_security_group_ids" {
   default     = []
 }
 
-//variable "db_subnet_group_name" {
-//  description = "Name of DB subnet group. DB instance will be created in the VPC associated with the DB subnet group. If unspecified, will be created in the default VPC"
-//  default     = ""
-//}
-//
+variable "db_subnet_group_name" {
+  description = "Name of DB subnet group. DB instance will be created in the VPC associated with the DB subnet group. If unspecified, will be created in the default VPC"
+  default     = ""
+}
+
 //variable "parameter_group_name" {
 //  description = "Name of the DB parameter group to associate"
 //  default     = ""