rbd: define image request layered flag

Define a flag indicating whether an image request is for a layered
image (one with a parent image to which requests will be redirected
if the target object of a request does not exist).  The code that
checks this flag will be added shortly.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>