qa/tasks/rbd: dev_create() expects a properties dict

qa/tasks/rbd: dev_create() expects a properties dict

In order to add encryption_format property, commit 3754c665a11e
(":qa/tasks/rbd: test qemu on top of rbd encryption") changed the
dev_create() contract.  It now expects a properties dict, but the
main task routine wasn't updated and still passes role_images.

On top of that, role_images itself got broken.  It is supposed
to be a mapping from role to a corresponding image name, not to
a dict with an image_name key.  This affected generic_mount().

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 0357c0215b2fbbf7fa801331df1f92b101ad9d5e)