Use a FullName:Tag representation when creating tarballs for (docker load)
Use a FullName:Tag representation when creating tarballs for (docker load)

This makes the tagging behavior consistent with (docker pull) for
projectatomic/docker, and does not change behavior for docker/docker.

See the added comment, and https://github.com/containers/image/issues/72 ,
for more explanation.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

masters-of-cats/image,containers/image,containers/image