weblogic.properties file has nothing to do with the web.xml and weblogic.xml
You should update the config.xml file in WL 8.1 installation with the details from weblogic.properties file. However I will suggest not tweaking the config.xml file manually, deploy your war file in WL 8.1 using hot deployment.
I believe WL 8.1 admin console provides user friendly ways to check whether the web application is properly deployed or not.
Let me know if you need more details.