Deprecate -f flag from docker tag

Closes #9798

@maintainers please note that this is a change to the UX. We no longer
require the -f flag on `docker tag` to move a tag from an existing image.
However, this does make us more consistent across our commands,
see https://github.com/docker/docker/issues/9798 for the history.

Signed-off-by: Doug Davis <dug@us.ibm.com>