Fix error message when stripping with format 1

Since the option '--format' for specifying the rbd image format is
deprecated, we should recommend '--image-format' instead.

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
Merge pull request #2688 from leseb/fix-warning-message-img-format

Fix error message when stripping with format 1

Reviewed-by: Loic Dachary <loic-201408@dachary.org>