bs_rbd.c: accept and parse bsopts

Options are supplied as one string, "key1=val1;key2=val2;" etc.

Legal options:
    conf=<conffile>
    id=<clientid>

For example, --bsopts "conf=/usr/local/ceph/ceph.conf;id=stgt"
(quoting is necessary to protect ; from the shell)

Unknown options are skipped.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>