title: Logback default value when runtime parameter is not supplied
body: <p>I have this conf in my logback.xml:</p>
  
  <pre><code>&lt;logger name="L1" additivity="false"&gt;
      &lt;appender-ref ref="${SELECTED_CONSOLE}" /&gt;
  &lt;/logger&gt;
  
  &lt;root level="${LOG_LEVEL}"&gt;
      &lt;appender-ref ref="${SELECTED_CONSOLE}" /&gt;
  &lt;/root&gt;
  </code></pre>
  
  <p>My issue is, on some env, if the LOG_LEVEL var hasn't been defined, what will the level of log of L1 be?</p>