cls_rbd: make sure stripe_unit is not larger than object size

Test a few other cases too.

backport: cuttlefish, bobtail
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
rbd.py: fix stripe_unit() and stripe_count()

These matched older versions of the functions, but would segfault
using the current versions.

backport: cuttlefish, bobtail
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
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>
Merge pull request #303 from ceph/wip-librbd-config-create

Reviewed-by: Sage Weil <sage.weil@inktank.com>