Create and use a JSON testenvironment file.

This is the format that TripleO-CI will be passing test environments
through to devtest, so by using it ourselves we avoid special casing
things quite so much.

Using a JSON testenvironment file also permits partially fixing a
FIXME in devtest.sh to stop sourcing some of the devtest scripts.

Change-Id: Ic3b0b482f411427ac8411591edc793e32401310f