Added reading OS variabled API_USERNAME and API_PASSWORD

[bbruun]

This was added to run ilo_exporter without showing credentials via "ps xa" by any user on a system running it as if it is started with -api.username=user -api.password=pass then ps xa |grep ilo_exporter will show credentials in cleartext so I added the few lines to enable running it as a systemd service or via a container without using the two parameters that divulge credentials.

Example:
In terminal 1:

$ ./ilo_exporter -api.username=ilo-exporter -api.password=password

In terminal 2:

$ ps xa |grep ilo
 416221 ?        Ssl    0:39 ./ilo_exporter -api.username=ilo-exporter -api.password=password

The systemd service file: /etc/systemd/system/ilo_exporter.service
(with a copy of ilo_exporter in /usr/local/bin/ with chown nobody:nobody and chmod +x)

[Service]
[Unit]
Description=HP iLO Prometheus exporter
Documentation=https://github.com/MauveSoftware/ilo_exporter Wants=network-online.target
After=network-online.target

[Install]
WantedBy=multi-user.target

[Service]
Type=simple
User=nobody
Group=nobody
ExecStart=/usr/local/bin/ilo_exporter -api.max-concurrent-requests=50 Restart=always

Systemd override conf file /etc/systemd/system/ilo_exporter.service.d/override.conf with chmod 600 and chown nobody:nobody

[Service]
Environment="API_USERNAME=ilo-exporter"
Environment="API_PASSWORD=password"