rgw: support partial mesh for zone sync

rgw: support partial mesh for zone sync

zone configuration now includes two new fields: sync_from_all
which is boolean, and sync_from, which is a least of zones to
sync from. By default sync_from_all is set to true. Sync will
only happen from all the zones, or from the specified zones if
sync_from all is false. We also check to see whether zone can
export data (depending on tier_type).

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>