qa/workunits/rbd: use create_image_and_enable_mirror() in bootstrap tests

qa/workunits/rbd: use create_image_and_enable_mirror() in bootstrap tests

The reason create_image() + enable_mirror() happens to work for
PARENT_POOL is that PARENT_POOL is enabled for mirroring in image mode
unconditionally, unlike POOL, POOL/NS1 or PARENT_POOL/NS1 for which
MIRROR_POOL_MODE setting is respected.  This isn't immediately obvious
because it's done in setup_pools() in rbd_mirror_helpers.sh.

Switch to create_image_and_enable_mirror() for clarity.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>