rbd image_read.sh: test different clone object orders

An rbd clone image can be created with an object order that differs
from that of its parent.  This patch adds testing for that in
qa/workunits/rbd/image_read.sh.

For simplicity, when a clone's object order differs from its parent
the order will be either one more than or one less than that of its
parent image, meaning its object size is either double or half of
the size of objects used in the parent.

Signed-off-by: Alex Elder <elder@inktank.com>