rbd: add --object-size option

rbd: add --object-size option

Object size can be specified when creating an image with the --order option,
as a number of bits in the size.

This patch is adding new option --object-size. This new option will specify
object size directly for example --object-size 2M.

It would be easier to use. --order is still present for backwards compatibility.
For simplicity, we are rounding up the object size to the nearest power of 2.

Fixes #12112

Signed-off-by: Vikhyat Umrao <vumrao@redhat.com>