Add alternate expression for ipmi down alert

[daaang]

This shares the same alert name, summary, labels, and for value with the existing up{job="ipmi"} == 0 alert. Sometimes when a lights out device is not responding, the ipmi exporter does respond like this:

# HELP ipmi_up '1' if a scrape of the IPMI device was successful, '0' otherwise.
# TYPE ipmi_up gauge
ipmi_up{collector="bmc"} 0
ipmi_up{collector="chassis"} 0
ipmi_up{collector="ipmi"} 0

If this happens, the corresponding up value will be 1 since there was a response with any metrics, even if that response happens to be "everything I tried to talk to is down."

So this adds a ipmi_up{collector="ipmi"} == 0 condition. It's fine for these to share a name in part because they will never fire at the same time:

1. If up == 0 then there are no metrics, so ipmi_up cannot also == 0.
2. If up == 1 then an alert only fires if ipmi_up is == 0.