Add support for gateways of type UXG to USG dashboard

[chrede88]

The USG dashboard doesn't currently support gateways of type uxg. At the time of opening this issue, I believe there are now three such gateways available: UXG-Lite, UXG-Max and UXG-Pro. I got some of the dashboard to work by simply including uxg as an option for the defined dashboard variables.

label_values(unpoller_device_info{type=~"udm|usg|ugw|uxg"},source)
label_values(unpoller_device_info{source=~"$Controller", type=~"udm|usg|ugw|uxg"},site_name)
label_values(unpoller_device_info{site_name=~"$Site", type=~"udm|usg|ugw|uxg"},name)

I'm running a UXG-Max, and can therefore run some tests if that's helpful.