Code review comment for lp:~mpontillo/maas/node-statistics-bug-1584926

This branch isn't quite done, but I wanted to put it up for review since it's getting quite large.

I've done the tests for the "nodes list" enhancements and "nodes by-type" operation.

Testing still left to do:
   - machines by-status [numeric=<bool>]
   - machines by-user
   - machines by-zone
   - devices by-parent [by_system_id=<bool>]

I'd hate to write all these tests if there is an issue with the API design, though.

I've got some examples here of how the API works:

    https://paste.ubuntu.com/17370652/