steps/infra: Use the bootstrap ignition in the tfvars if its set.

If the "ignition_bootstrap" is non-empty in the terraform.tfvars,
then use it as the content for the bootstrap ignition.

This will enable the new installer binary to launch a cluster.

config.tf

@@ -312,6 +312,15 @@ variable "ignition_worker" {
 EOF
 }
 
+variable "ignition_bootstrap" {
+  type    = "string"
+  default = ""
+
+  description = <<EOF
+(internal) Ignition config file contents. This is automatically generated by the installer.
+EOF
+}
+
 variable "tectonic_platform" {
   type = "string"
 

steps/infra/aws/inputs.tf

@@ -7,5 +7,5 @@ data "terraform_remote_state" "assets" {
 }
 
 locals {
-  ignition_bootstrap = "${data.terraform_remote_state.assets.ignition_bootstrap}"
+  ignition_bootstrap = "${var.ignition_bootstrap != "" ? var.ignition_bootstrap : data.terraform_remote_state.assets.ignition_bootstrap}"
 }

steps/infra/libvirt/inputs.tf

@@ -7,5 +7,5 @@ data "terraform_remote_state" "assets" {
 }
 
 locals {
-  ignition_bootstrap = "${data.terraform_remote_state.assets.ignition_bootstrap}"
+  ignition_bootstrap = "${var.ignition_bootstrap != "" ? var.ignition_bootstrap : data.terraform_remote_state.assets.ignition_bootstrap}"
 }