rbd: respect rbd_default_* parameters

rbd: respect rbd_default_* parameters

Treat rbd_default_{format,order,stripe_unit,stripe_count} as defaults for
the usual arguments for specifying those properties.

librbd::create() is affected by rbd_default_format, so we need to
explicitly override it if --image-format is set. The rest of the
parameters are passed explicitly when they are used, so their rbd_default
equivalents don't matter.

Fixes: #8821
Signed-off-by: Josh Durgin <josh.durgin@inktank.com>