Add log4j2.xml to forwarder package.

[malcyon]

This ensures that users can configure logging via the log4j2.xml file. It also directs logs to a file instead of to the journal by default.

The log4j2.xml file will exist here:

• /etc/graylog/forwarder/log4j2.xml

By default, logs will be written to a file here:

• /var/log/graylog-forwarder/forwarder.log