rgw: rgw-admin throw an error when invalid flag is passed

fix #5820 http://tracker.ceph.com/issues/5820

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>
Merge pull request #495 from kri5/wip-5820

rgw: rgw-admin throw an error when invalid flag is passed

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>