Clean up data extraction

Based on input from @eglynn make adjustments to how data
is extracted.

Note that I'm doing this TDD, so the test file is authoritative
on what is expected.

* resource is now composed from node_uuid and sensor id
* meter name is simplified, but includes ipmi
* unit is parsed out of sensor data

Questions:

* We now have a generic meter name for readings from 11 different
  sensors. Is this how it is supposed to work? What does a "meter
  name" mean?