title: How to run Terraform without specify provider?
body: <p>I was wondering if it is possible to run <code>terraform</code> without specifying the <code>provider</code> block, and instead rely purely on environmental variables?</p>
  
  <p>Like this:</p>
  
  <pre><code>#!/bin/bash
  export TF_VAR_user_domain_name="foo"
  export TF_VAR_tenant_id="faa"
  export TF_VAR_tenant_name="fii"
  export TF_VAR_user_name="name"
  export TF_VAR_region="region"
  export TF_VAR_auth_url="url"
  </code></pre>
  
  <p>Instead of :</p>
  
  <pre><code>provider "openstack" {
    user_name   = "admin"
    tenant_name = "admin"
    password    = "pwd"
    auth_url    = "url"
    region      = "RegionOne"
  }
  </code></pre>