Include a 'node' key and value in ipmi metadata

The ipmi meters use resource_ids which make it challenging
to select information for all the sensors of a particular type.
This change adds metadata that identifies the physical node on
which a sensor exists.

There are still remaining issues with the naming of ipmi and
sensor-related meters that will need to be addressed separately.

Closes-Bug: 1377157
Change-Id: Ib53755112e4757aa53ab60079f3c555e21a72b63
(cherry picked from commit 3b20fa08527170aaca38253277feedf0f2d11217)
Merge "Include a 'node' key and value in ipmi metadata" into proposed/juno