Adopt Terraform v0.12 templatefile function

* Update terraform-render-bootstrap module to adopt the
Terrform v0.12 templatefile function feature to replace
the use of terraform-provider-template's `template_dir`
* Require Terraform v0.12.6+ which adds `for_each`

Background:

* `template_dir` was added to `terraform-provider-template`
to add support for template directory rendering in CoreOS
Tectonic Kubernetes distribution (~2017)
* Terraform v0.12 introduced a native `templatefile` function
and v0.12.6 introduced native `for_each` support (July 2019)
that makes it possible to replace `template_dir` usage

bare-metal/fedora-coreos/kubernetes/bootstrap.tf

@@ -1,6 +1,6 @@
 # Kubernetes assets (kubeconfig, manifests)
 module "bootstrap" {
-  source = "git::https://github.com/poseidon/terraform-render-bootstrap.git?ref=0daa1276c633fea28e41b2c2c18831e2584deb24"
+  source = "git::https://github.com/poseidon/terraform-render-bootstrap.git?ref=1bba891d95b143866614c928f87026e4c79f2eae"
 
   cluster_name                    = var.cluster_name
   api_servers                     = [var.k8s_domain_name]

bare-metal/fedora-coreos/kubernetes/versions.tf

@@ -1,7 +1,7 @@
 # Terraform version and plugin versions
 
 terraform {
-  required_version = "~> 0.12.0"
+  required_version = "~> 0.12.6"
   required_providers {
     matchbox = "~> 0.3.0"
     ct       = "~> 0.4"