* Without this change, if a cluster doesn't set the controller
or worker networkd lists, an err "element() may not be used
with an empty list" occurs.
* controller_networkds and worker_networks are intended to be
optional and temporary, not required at all
* Accept ordered lists of controller and worker networkd configs
* Do not rely on these variables. They will be replaced with a
cleaner mechanism at a future date
* Template bare-metal Container Linux configs with Terraform's
(limited) template_file module. This allows rendering problems
to be identified during `terraform plan` and is favored over
using the Matchbox templating feature when the configs are
served to PXE booting nodes.
* Writes a Matchbox profile for each machine, which will be served
as-is. The effect is the same, each node gets provisioned with its
own Container Linux config.
