title: Input variable for terraform provider version
body: <p>In a CI/CD context, I would like to define provider versions outside my terraform configuration using <code>TF_VAR_</code> <a href="https://www.terraform.io/docs/configuration/variables.html#environment-variables" rel="nofollow noreferrer">environment variables</a>.</p>
  <p>I'm trying to use input variable to set the version of helm provider in <code>versions.tf</code> (terraform 0.12) but it seems not allowed :</p>
  <pre><code>Error: Invalid provider_requirements syntax
  
    on versions.tf line 3, in terraform:
     3:     helm = &quot;${var.helm_version}&quot;
  
  provider_requirements entries must be strings or objects.
  
  
  Error: Variables not allowed
  
    on versions.tf line 3, in terraform:
     3:     helm = &quot;${var.helm_version}&quot;
  
  Variables may not be used here.
  </code></pre>
  <p>How can I configure this ?</p>
  <p>If it's not possible, how I can manage the terraform provider version outside my configuration ?</p>