Insert generated image.json into the ostree commit [retry]

Retry landing this since a followup commit should fix AWS replication.

This is part of coreos/fedora-coreos-tracker#1151

Our generated disk images are largely just a "shell" around the egg
of an ostree commit.  There is almost nothing that lives
in the disk image that isn't in the commit.

(This is especially true now that a preparatory commit previous to
 this moved the *content* of our static `grub.cfg` into `image.json`)

In the original coreos-assembler design I'd tried to cleanly
separate builds of the ostree from disk image builds, but also
support linking them together (with matching version numbers, etc.)
The separate `image.yaml` was part of this.  This...mostly worked.

This change furthers that separation by having image builds input from
*just the ostree commit*.  Crucially we would no longer need the config
git repository to perform an image build.

And this in turn unlocks truly better separating ostree builds from
disk image builds in the pipeline *and* supporting
downstream tooling generating disk images from custom containers.

One neat thing here is we will finally fix a longstanding issue
where coreos-assembler fails when just the `image.yaml` changes:

Closes: coreos#972

src/cosalib/qemuvariants.py

@@ -183,6 +183,9 @@ def __init__(self, **kwargs):
 
         _Build.__init__(self, **kwargs)
 
+        repo = os.path.join(self._workdir, 'tmp/repo')
+        import_ostree_commit(repo, self._tmpdir, self.meta)
+
     @property
     def image_qemu(self):
         """