rgw: deny writes to a secondary zone by non-system users

Fixes: #6678
We don't want to allow regular users to write to secondary zones,
otherwise we'd end up with data inconsistencies.

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
v0.72