Use boolean value in condition (#148)

This PR is a follow-up of PR #118.

This is the exact same changeset than #137 but for the `create_db_subnet_group`
variable.

main.tf

 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}"
+  enable_create_db_subnet_group = "${var.db_subnet_group_name == "" ? var.create_db_subnet_group : false}"
 
   parameter_group_name    = "${coalesce(var.parameter_group_name, var.identifier)}"
   parameter_group_name_id = "${coalesce(var.parameter_group_name, module.db_parameter_group.this_db_parameter_group_id)}"