rbd.cc: add readonly option for "rbd map"

The device could only be set to rw(default) when mapping
now. This patch only handle the user space, because the
kernel part has been completed.

Signed-off-by: Guangliang Zhao <guangliang@unitedstack.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Merge git://github.com/ceph/ceph