aws: Add Ingress ELB DNS name output as ingress_dns_name

* Expose the Ingress ELB DNS name so application DNS records can
be defined in Terraform to resolve to the Ingress ELB

aws/container-linux/kubernetes/ingress.tf

@@ -18,7 +18,7 @@ resource "aws_elb" "ingress" {
     instance_protocol = "tcp"
   }
 
-  # Kubelet HTTP health check
+  # Ingress Controller HTTP health check
   health_check {
     target              = "HTTP:10254/healthz"
     healthy_threshold   = 2

aws/container-linux/kubernetes/outputs.tf

@@ -0,0 +1,5 @@
+output "ingress_dns_name" {
+  value = "${aws_elb.ingress.dns_name}"
+  description = "DNS name of the ELB for distributing traffic to Ingress controllers"
+}
+