Cleanup:change "latest" to graph.DEFAULTTAG

Signed-off-by: Lei Jitang <leijitang@huawei.com>

I don't know if this is necessary or not,if not, feel free to close this.
But for a good coding style, it's better to use a global constant.
And some place also use `grath.DEFAULTTAG` instead of `latest`
https://github.com/docker/docker/blob/master/daemon/create.go#L43