I think the descriptor you are using is out of date. We no longer specify the configId and parentId in the <webapp> element. You can use this sample application as a reference: http://cwiki.apache.org/GMOxDOC21/web-application-security-sample.html.
The key is
<environment>
<moduleId>
<groupId>org.apache.geronimo.samples</groupId>
<artifactId>timereport-war</artifactId>
<version>2.1</version>
<type>war</type>
</moduleId>
</environment>
Thanks,
Viet
