rbd: add 'config_info' sysfs rbd device attribute

Export the info used to setup the rbd image, so it can be used to remap
the image.

Signed-off-by: Mike Christie <mchristi@redhat.com>
[idryomov@gmail.com: do_rbd_add() EH]
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>