Add cpu-{node,mem,disk} to the testenv JSON

devtest in Ibc61c8e062833d56090559057d8b3aa38a42d275 will expect to see
node-cpu, node-mem, and node-disk in the JSON datafile, so add the
defaults in.

Change-Id: I079af4a9c27d2a42e776b8264a51360d7ff503f8