rgw: Match decode_json with dump for default-placement in RGWZoneGroup.

[xijiacun]

Fixes: https://tracker.ceph.com/issues/54016
Signed-off-by: zhiming zhang zhangzhm1@chinatelecom.cn

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[xijiacun]

@cbodley Please help to review the code. Thanks

[cbodley]

passed qa https://pulpito.ceph.com/cbodley-2022-01-27_22:43:41-rgw-wip-cbodley-testing2-distro-default-smithi (rerun https://pulpito.ceph.com/cbodley-2022-01-28_15:14:53-rgw-wip-cbodley-testing2-distro-default-smithi/)