When performing a tag, if a shortname was provided, tag.go would
preprend docker.io to the shortname through the ParseNormalized
function. Here we work around that such that is a short name
and tag are provided, the resulting tag will be shortname:tag. If
a shortname is provided without a tag, we append "latest" as the
tag.
Added specific tag tests too
Signed-off-by: baude <bbaude@redhat.com>
