rbd: small changes

A few very minor changes to the rbd code:
    - RBD_MAX_OPT_LEN is unused, so get rid of it
    - Consolidate rbd options definitions
    - Make rbd_segment_name() return pointer to const char

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