variable "instance_type" {
  type = "map"

  default = {
    test = "m4.large"
    uat  = "m4.large"
    prod = "m4.xlarge"
  }
}
// <...>

module "ecs_cluster" {
  // <...>
  instance_type                        = "${lookup(var.instance_type, terraform.workspace)}"
}