qa rbd image_read: a few cleanups

The function boolean_toggle() in qa/workunits/rbd/image_read.sh is
defined but never used.  My intentions were good though.  Fix it and
use it for argument parsing.

Change the minimum supported object order so it matches what the
command line interface enforces.

Assign the initial value of TEST_CLONES from the environment if it's
available.

Make the order of assignment of argument variables consistent.

And fix a typo unmap_image().

Signed-off-by: Alex Elder <elder@inktank.com>
rbd qa image_read.sh: allocate local file at create time

Move the dd command that touches the last byte in a local file
into create_image() where it belongs (out of fill_original()).

Signed-off-by: Alex Elder <elder@inktank.com>
rbd image_read.sh: add clone-of-clone testing

Add testing to verify that a snapshot of a clone and a clone of
that snapshot both produce the correct results when read.

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