rbd: du command should take spec as an argument

rbd: du command should take spec as an argument

Change du to take <image-spec> | <snap-spec> as an argument instead of
going through --image option.  The new synopsis is

    (du | disk-usage) [<image-spec> | <snap-spec>]

This is to make it look more like the rest of the commands: the only
other command that takes pool as an argument is ls and it can't really
serve as a prototype for du, because the latter has to work on images
and snapshots as well.

Examples:

    # stats for pool rbd
    $ rbd du
    $ rbd -p rbd du

    # stats for pool foo
    $ rbd -p foo du

    # stats for snapshot mysnap of image baz in pool rbd
    $ rbd du baz@mysnap

    # stats for image bar in pool foo
    $ rbd du foo/bar

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