Added support for role based login for Fritz-Boxes

Since FRITZ!OS 5.50 it is possible to use role based security (so you can add a dedicated user "nagios" for monitoring with less rights). 

This pull requests allows to use --username for specifying the user if this mode is used.

Thanks btw for the hot chili sauce, it is really delicious (I tried it yesterday in my train marathon since I had nothing else to eat ;-) BTW, it is really hot ;-D