[Terraform] [AWS] Find a way to enable/disable the exchange module via a terraform variable

Due to my lack of knowledge around terraform, I suspect [my method of enabling/disabling the Exchange module](https://www.detectionlab.network/customization/exchange-server/#aws) is suboptimal. It would be much better if this was controlled via a variable in `terraform.tfvars` so changes to the terraform files didn't causes changes in git to become staged.