job-archive-interface: clean up which data is fetched for job-usage calculation

[cmoussa1]

Broken off from a comment in #353, the data fetched by job-archive-interface.py could probably use some cleanup. There are some attributes it's currently fetching from flux-core's job-archive DB that it probably doesn't need anymore (e.g job-list returns nnodes now). I should look to improve what data is gathered both from job-list and job-info so that the table that stores the job records in the flux-accounting DB is

1. as organized as possible, and
2. specific to the needs of flux-accounting

[cmoussa1]

#357 now adds a specific jobs table to the flux-accounting database with the data it requires for job usage calculations, so I think I can safely close this. Will re-open more specific issues if we find that we need other kinds of data not already captured.