rbd: add rbd default map options config option

rbd: add rbd default map options config option

Allow supplying rbd map -o / --options through 'rbd default map
options' config option.  Options specified on the command line take
precedence on a per-option basis, i.e. default and cli options are
merged with a preference for the latter.  For example:

    # grep 'rbd default map options' ceph.conf
            rbd default map options = "ro,nocrc"
    # ./rbd map -o noshare,rw foo
    /dev/rbd0

results in a rw mapping with nocrc,noshare.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>