terraform: remove modules in repo (hashicorp#5085)

* terraform: remove modules in repo

These are not currently being maintained and tested, and were
created prior to the existence of the Terraform Module Registry,
which is the more appropriate way to share and distribute modules.

In an effort to limit confusion of the purpose of these modules and
not encourage usage of something we aren't confident about, this
removes them from this repository.

You can still access these modules if you depend on them by pinning to
a specific ref in Git.

It is recommended you pin against a recent major version where
these modules existed:

```
module "consul-aws" {
  source = "git::https://github.com/hashicorp/consul.git//terraform/aws?ref=v1.4.0"
}
```

More detail about module sources can be found on this page:

https://www.terraform.io/docs/modules/sources.html

* terraform: add a readme for anyone who can't find the modules

Author: pearkes

terraform/README.md

@@ -1,7 +1,14 @@
-# Terraform Modules
+These Terraform modules were removed from GitHub in [GH-5085](https://github.com/hashicorp/consul/pull/5085).
 
-This folder contains modules for Terraform that can setup Consul for
-various systems. The infrastructure provider that is used is designated
-by the folder above. See the `variables.tf` file in each for more documentation. 
+These are not currently being maintained and tested, and were created prior to the existence of the Terraform Module Registry, which is the more appropriate way to share and distribute modules.
 
-To deploy Consul in multiple Subnets/AZ on AWS - supply:  -var 'vpc_id=vpc-1234567' -var 'subnets={ "0" = "subnet-12345", "1" = "subnet-23456", "2" = "subnet-34567"}'
+In an effort to limit confusion of the purpose of these modules and not encourage usage of something we aren't confident about, this removes them from this repository.
+
+You can still access these modules if you depend on them by pinning to a specific ref in Git. It is recommended you pin against a recent major version where these modules existed:
+
+module "consul-aws" {
+  source = "git::https://github.com/hashicorp/consul.git//terraform/aws?ref=v1.4.0"
+}
+More detail about module sources can be found on this page:
+
+https://www.terraform.io/docs/modules/sources.html