Monitoring: add collectd-mod-sensors

[FFHener]

Example at Saarbruecker:
https://monitor.berlin.freifunk.net/cgp/host.php?h=saarbruecker-gw&p=sensors

We should at least activate this at the gateways.

[LipkeGu]

For this you need lm-sensors and lm-sensors-detect (it adds alot of dependencies) not every device exposes sensors.
sensors should be added to the model file. keep in mind not every device exposes sensors.

something like:

sensors:
  -coretemp
  -ias8266xyz...