External configuration for prometheus-libvirt-exporter.service

[fprzewozny]

Hey,
prometheus-libvirt-exporter allows for some configuration to be passed as arguments (the most important for me is web.listen-address) which is great, but if systemd service is used (prometheus-libvirt-exporter.service) then the only way to change execution arguments is to change the .service file (which may be fine in small scale, without configuration management). It would be really useful if some kind of configuration file was present outside prometheus-libvirt-exporter.service file (f.e. /etc/prometheus-libvirt-exporter or /etc/default/prometheus-libvirt-exporter).

Please let me know @frittentheke what do you think about it.

Thank you,
Franciszek