ceph_argparse.py: add stderr note if nonrequired param is invalid

ceph_argparse.py: add stderr note if nonrequired param is invalid

If we run across a user-supplied parameter that doesn't validate against
a non-required descriptor, it may be that it's a valid entry for a later
descriptor...or it may be that it's supposed to match.  We can't really tell.
A possible heuristic would be to call it invalid-for-sure if we're at the
end of the descriptor list, but that's not very generic.

Warn about it and try to drive on anyway.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>