rgw: Adds --rgw-zone --rgw-region help text.

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>
Merge pull request #448 from kri5/wip-5416

rgw: Adds --rgw-zone --rgw-region help text.

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>