* 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
* Define bare-metal `controllers` and `workers` as a complex type
list(object{name=string, mac=string, domain=string}) to allow
clusters with many machines to be defined more cleanly
* Remove `controller_names` list variable
* Remove `controller_macs` list variable
* Remove `controller_domains` list variable
* Remove `worker_names` list variable
* Remove `worker_macs` list variable
* Remove `worker_domains` list variable
Dalton Hubble