Avoid new cert / ELB error

There is a race condition in Terraform when it creates an ELB with a newly created certificate:

```
* aws_elb.concourse: [WARN] Error creating ELB Listener with SSL Cert, retrying: CertificateNotFound: Server Certificate not found for the key: arn:aws:iam::xxxx:server-certificate/xxxx
	status code: 400, request id: xxxx
```
Here we add a sleep to give time to the IAM certificate to propagate.
This has already been raised: hashicorp/terraform#3275

terraform/concourse/elb.tf

@@ -2,6 +2,10 @@ resource "aws_iam_server_certificate" "concourse" {
   name = "${var.env}-concourse"
   certificate_body = "${file("concourse.crt")}"
   private_key = "${file("concourse.key")}"
+
+  provisioner "local-exec" {
+    command = "sleep 10"
+  }
 }
 
 resource "aws_elb" "concourse" {