Fix the bug of set an existed tag name of a repository

When tag a image with a tag name which is existed in the repository,
docker should prompt it is conflicted and it will worked when with
a -f option.But the docker doesn't work like that,it will just
replace the image whether you have a -f option or not.View the
code you can find that if an repoName is existed,it will never go
to check if 'force' option is set or not.


Signed-off-by: Lei Jitang <leijitang@huawei.com>
I not sure if this is right,but the docker commit and docker import command also set the 'force' flag to 'true'.

if repository != "" {
                if err := daemon.repositories.Set(repository, tag, img.ID, true); err != nil {
                        return img, err
                }
        }