Add alb_name variable to specify load balancer name

According to AWS docs [0], the Load Balancer name is limited to 32
characters. Similar to cloudposse#27, this diff adds an `alb_name` variable
to avoid using the default label id, if set.

[0] https://docs.aws.amazon.com/elasticloadbalancing/2012-06-01/APIReference/API_CreateLoadBalancer.html

main.tf

@@ -61,7 +61,7 @@ module "access_logs" {
 
 resource "aws_lb" "default" {
   count              = module.this.enabled ? 1 : 0
-  name               = module.this.id
+  name               = var.alb_name == "" ? module.this.id : var.alb_name
   tags               = module.this.tags
   internal           = var.internal
   load_balancer_type = "application"

variables.tf

@@ -14,6 +14,12 @@ variable "security_group_ids" {
   description = "A list of additional security group IDs to allow access to ALB"
 }
 
+variable "alb_name" {
+  type        = string
+  default     = ""
+  description = "The name for the load balancer, uses a module label name if left empty"
+}
+
 variable "internal" {
   type        = bool
   default     = false