Render config template, substituting grains where found.

def _render_template(config_file):
    '''
    Render config template, substituting grains where found.
    '''
    dirname, filename = os.path.split(config_file)
    env = jinja2.Environment(loader=jinja2.FileSystemLoader(dirname))
    template = env.get_template(filename)
    return template.render(__grains__)