Compose the OSTree in the mock container

def ostree_compose(self, release):
        """Compose the OSTree in the mock container"""
        start = datetime.utcnow()
        treefile = os.path.join(release['git_dir'], 'treefile.json')
        cmd = release['ostree_compose'] % treefile
        with file(treefile, 'w') as tree:
            json.dump(release['treefile'], tree)
        # Only use new_chroot for the invocation, as --clean and --new-chroot are buggy together right now
        out, err, rcode = self.mock_chroot(release, cmd, new_chroot=True)
        ref = None
        commitid = None
        for line in out.split('\n'):
            if ' => ' in line:
                # This line is the: ref => commitid line
                line = line.replace('\n', '')
                ref, _, commitid = line.partition(' => ')
        self.log.info('rpm-ostree compose complete (%s), ref %s, commitid %s',
                      datetime.utcnow() - start, ref, commitid)
        return ref, commitid