New function: single-node-asg module supports binding EIP by itself.

Since it is single node, binding an EIP to the instance is possible. And
it eases other things since the public interface is constant.

Add assign_eip variable to single-node-asg. If turns it on, an EIP will
be allocated, and assocated with the instance.

modules/single-node-asg/main.tf

@@ -85,13 +85,13 @@ module "server" {
   elb_names          = var.load_balancers
   key_name           = var.key_name
   # The IAM Instance Profile w/ attach_ebs role
-  iam_profile   = module.instance_profile.iam_profile_id
-  instance_type = var.instance_type
+  iam_profile        = module.instance_profile.iam_profile_id
+  instance_type      = var.instance_type
   # 1 EC2 instance <> 1 EBS volume
-  max_nodes       = 1
-  min_nodes       = 1
-  placement_group = var.placement_group
-  public_ip       = var.public_ip
+  max_nodes          = 1
+  min_nodes          = 1
+  placement_group    = var.placement_group
+  public_ip          = var.public_ip
   # the prefix and suffix names are combined in
   # the `asg` module to create the full name
   name_prefix        = var.name_prefix