modules/aws/bootstrap: Add elbs_length

To work around Terraform's limitations on length() of computed lists
at plan-time [1].  By avoiding the generated resources and just
computing the number of elastic load-balancers based on our input
counts, we avoid confusing Terraform and getting [2]:

  * module.bootstrap.aws_elb_attachment.bootstrap:
    aws_elb_attachment.bootstrap: value of 'count' cannot be computed

The value of 2 comes from:

* One internal LB (either aws_elb.api_internal or aws_elb.api_external) and
* One aws_elb.console.

[1]: hashicorp/terraform#12570 (comment)
[2]: https://storage.googleapis.com/origin-ci-test/pr-logs/pull/openshift_installer/268/pull-ci-openshift-installer-e2e-aws/316/build-log.txt

modules/aws/bootstrap/main.tf

@@ -121,7 +121,7 @@ resource "aws_instance" "bootstrap" {
 }
 
 resource "aws_elb_attachment" "bootstrap" {
-  count    = "${length(var.elbs)}"
+  count    = "${var.elbs_length}"
   elb      = "${var.elbs[count.index]}"
   instance = "${aws_instance.bootstrap.id}"
 }

modules/aws/bootstrap/variables.tf

@@ -24,6 +24,10 @@ variable "elbs" {
   description = "Elastic load balancer IDs to attach to the bootstrap node."
 }
 
+variable "elbs_length" {
+  description = "The length of the 'elbs' variable, to work around https://github.com/hashicorp/terraform/issues/12570."
+}
+
 variable "iam_role" {
   type        = "string"
   default     = ""

modules/aws/vpc/outputs.tf

@@ -46,6 +46,10 @@ output "aws_lbs" {
   value = ["${compact(concat(aws_elb.api_internal.*.id, list(aws_elb.console.id), aws_elb.api_external.*.id))}"]
 }
 
+output "aws_lbs_length" {
+  value = "2"
+}
+
 output "aws_elb_api_external_dns_name" {
   value = "${element(concat(aws_elb.api_external.*.dns_name, list("")), 0)}"
 }

steps/infra/aws/main.tf

@@ -23,6 +23,7 @@ module "bootstrap" {
   bucket                      = "${aws_s3_bucket.tectonic.bucket}"
   cluster_name                = "${var.tectonic_cluster_name}"
   elbs                        = "${module.vpc.aws_lbs}"
+  elbs_length                 = "${module.vpc.aws_lbs_length}"
   iam_role                    = "${var.tectonic_aws_master_iam_role_name}"
   ignition                    = "${local.ignition_bootstrap}"
   subnet_id                   = "${module.vpc.master_subnet_ids[0]}"