title: Terraform: "known only after apply" ISSUE
body: <p>I'm creating an <code>aws_subnet</code> and referencing it in another resource.</p>
  <p>Example:</p>
  <pre><code>resource &quot;aws_subnet&quot; &quot;mango&quot; {
       vpc_id     = aws_vpc.mango.id
       cidr_block = &quot;${var.subnet_cidr}&quot;
    }
  </code></pre>
  <p>The reference</p>
  <pre><code> network_configuration {
      subnets          = &quot;${aws_subnet.mango.id}&quot;
    }
  </code></pre>
  <p>When planning it I get
  <code>aws_subnet.mango.id is a string, known only after apply</code>
  error. I'm new to Terraform. Is there something similar to Cloudformation's <code>DependsOn</code> or <code>Export/Import</code>?</p>