RGW - Zipper - Make Zone generic

RGW - Zipper - Make Zone generic

- RadosStore had a dummy implementation of Zone that just passed through
  to the Zone service to get info about the local zone.  Implement the
  Zone API for real, so that we can reference multiple zones.
- Convert get_id() to string; it was only ever used as a string anyway.
- Add get_zone_by*() to zonegroup and use it apporpriately

Signed-off-by: Daniel Gryniewicz <dang@redhat.com>