Use the JSON description to probe the seed VM.

This adds a dependency on jq to let us cleanly write autodetected seed
VM IP addresses into the test file, which will simplify things later
on in our workflow. Using the datafile is preferrable to complex
checks in devtest which is afterall meant to be as much manual as
script.

Change-Id: I81e49d16077c564dd1e5d3e6f6e7a26e7c56ef5c
Use the test env JSON to detect remote VM hosts.

This permits devtest_seed to automatically do the right thing in a
TripleO-CI environment. Well, nearly - we still need to fixup some of
the post boot steps, but we should at least have a seed booting from
this.

Change-Id: I1fedc9e41ba2eec96bd21ec618ff48a678a383ce