daemon: resolve the graphdriver to show

graphdriver is not always specified when the log printed, because
it's provided in another thread. This patch will fix this.

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
@crosbymichael right now we can get these information from two ways: the `daemon` method and the daemonCfg, if we want to print this information in mainDaemon(), I'm not sure how we can use a method to get that. And daemonCfg.GraphDriver is out dated in NewDaemonFromDirectory(), so I think update this is reasonable.What do you think?