Recursively traverse vehicle health report.

def _traverse_report(data):
    """Recursively traverse vehicle health report."""
    if 'items' not in data:
        return {}
    out = {}
    for item in data['items']:
        skip = (item['severity'] == 'NonDisplay' or
                item['itemKey'] == 'categoryDesc' or
                item['value'] in [None, 'Null', 'N/A', 'NULL'])
        if skip:
            continue
        value = 'Ok' if item['value'] == '0.0' else item['value']
        out[item['itemKey']] = value
        out.update(_traverse_report(item))
    return out