librbd: make image creation defaults configurable

librbd: make image creation defaults configurable

Programs using older versions of the image creation functions can't
set newer parameters like image format and fancier striping.

Setting these options lets them use all the new functionality without
being patched and recompiled to use e.g. rbd_create3().
This is particularly useful for things like qemu-img, which does not
know how to create format 2 images yet.

Refs: #5067
backport: cuttlefish, bobtail
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>