rbd image_read.sh: 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.

Change defaults to use format 2 and test clones.

Output details about the parameters of the run even if not being
verbose.

Make the order of assignment of argument variables consistent.

And fix a typo unmap_image().

Signed-off-by: Alex Elder <elder@inktank.com>
rbd 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>
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.  By default, clone images will be
created with an object size twice as big as that of its parent.

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>