rgw: Adds --system option help to radosgw-admin

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>
Merge pull request #528 from kri5/wip-radosgw-admin-help

rgw: Adds --system option help to radosgw-admin

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>