Store node details in the JSON datafile

Rather than storing environment variables that show what detail nodes
we may have created have (which we don't do for CI anyway), store the
details of nodes created in the JSON datafile, and make use of those
details in setup-baremetal, if they're set, falling back to the current
environment variables, if they are not.

Change-Id: Ibc61c8e062833d56090559057d8b3aa38a42d275
Co-Authored-By: Robert Collins <rbtcollins@hp.com>
Merge "Store node details in the JSON datafile"