Fix tag an existed tag name of a repository

replace #8303 
When tag a image with a tag name which is existed in the repository,
docker should prompt it is conflicted and it will only 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.
@LK4D4 you mean I alse should check if err is nil or not? 
if !strings.Contains(out, "Conflict: Tag test is already set to image") && err != nil  ?