Expose PSU redundancy metrics

[aieri]

The IPMI sensors collector is already exposing metrics about power supplies, but they could be surfaced a bit better.

This is how they currently look:

ubuntu@wooster:~$ curl -s localhost:10000 | grep -i 'power supply'
ipmi_generic_sensor_value{name="PS1 Status",state="Nominal",type="Power Supply",unit="N/A"} 0.0
ipmi_generic_sensor_value{name="PS2 Status",state="Nominal",type="Power Supply",unit="N/A"} 0.0

We could take these values out of the generic section and have a dedicated metric:

ipmi_power_supply_presence{name="PS1 Status",state="Present"} = 1.0
ipmi_power_supply_presence{name="PS2 Status",state="Absent"} = 0.0

Even better, we could have a redundancy metric:

ipmi_power_supply_redundancy = 1.0 # with some arbitrary meaning of 1 = ok, 0 = no redundancy, 2 = redundancy lost

This metric would not come 1:1 from ipmi-sensors but could be derived by the whether power supply sensors are present, you have more than 2, and there are at least 2 up.