rgw: fix rgw test to reflect usage change

otherwise src/test/cli/radosgw-admin/help.t fails when running make
check when run after a configure --with-radosgw

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #627 from dachary/master

rgw: fix rgw test to reflect usage change

Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>