title: Define PM2 script path with environment variable
body: <p>I'm building a simple staging CI environment where I define the path of the runnable Node application by an environment variable APP_PATH.</p>
  
  <p>Is it possible to tell PM2 in a process config file to look up the startup script by following the path in the APP_PATH variable?</p>
  
  <p>Like this:</p>
  
  <pre><code>{
    "apps": [
      {
        "name": "my_app",
        "script": "$APP_PATH/app.js"
      }
    ]
  }
  </code></pre>
  
  <p>Currently, I get a <code>[PM2][ERROR] script not found :</code> message from PM2 when starting the above configuration.</p>
  
  <p><strong>In a nutshell:</strong> PM2 doesn't resolve the env var defined in the <code>script</code> property. Is there any way to overcome this?</p>