No need to add 'session-config' node when already existing from template

With previous code, web.xml generation fails when session timeout value is provided with this exception:

```java.lang.IllegalArgumentException: <session-config> element is limited to 1 occurrence```

This is because the new template for web.xml file already contains that node and the plugin adds a new one for timeout configuration.

The change I made was for only modifying the timeout value when it already exists and add it when it doesn't (with the same code as before).