title: Variable keys in terraform maps
body: <p>In Terraform, I'm trying to create a module with that includes a map with variable keys.  I'm not sure if this is possible but I've tried the following without success.</p>
  
  <pre class="lang-tf prettyprint-override"><code>resource "aws_instance" "web" {
      ami = "${var.base_ami}"
      availability_zone = "${var.region_a}"
      instance_type = "${var.ec2_instance_size}"
      security_groups = ["sec1"]
      count = "${var.ec2_instance_count}"
      tags {
          Name = "${var.role} ${var_env}"
          role = "${var.app_role}"
          ${var.app_role} = "${var_env}"
      }
  }
  </code></pre>
  
  <p>and this:</p>
  
  <pre class="lang-tf prettyprint-override"><code>tags {
     Name = "${var.role} ${var_env}"
  }
  tags."${var.role}" = "${var.env}"
  </code></pre>
  
  <p>Any ideas? Is this not possible with Terraform currently?</p>