modules/aws/etcd/ignition: Drop hard-coded ignition_etcd_keys

The local variable is from cd71f30 (Ignition assets, 2018-02-14,
coreos/tectonic-installer#2940), but it doesn't scale beyond three
etcd nodes.  This commit pivots to an approach that scales to an
arbitrary number of nodes, matching the scalable generation logic
we've had since 0c7ac90 (modules/*: trust CA certificates on the
nodes, 2017-11-22, coreos/tectonic-installer#2362).

modules/aws/etcd/ignition.tf

@@ -1,7 +1,3 @@
-locals {
- ignition_etcd_keys = ["ignition_etcd_0.json", "ignition_etcd_1.json", "ignition_etcd_2.json"]
-}
-
 data "ignition_config" "tnc" {
  count = "${var.instance_count}"
 
@@ -13,7 +9,7 @@ data "ignition_config" "tnc" {
 
  # Used for loading certificates
  append {
- source = "${format("s3://%s/%s", var.s3_bucket, local.ignition_etcd_keys[count.index])}"
+ source = "${format("s3://%s/ignition_etcd_%d.json", var.s3_bucket, count.index)}"
 
  # TODO: add verification
  }