rgw/multisite: assign 'rgw_zone' to the exact section in ceph.conf

rgw/multisite: assign 'rgw_zone' to the exact section in ceph.conf

since the following commit:
  commit 1ac94c048ff1d1385de2892d0ecef7879ec563e9
  rgw: add support for multiple rgw instances on a single host

we have multi-instance rgw support on a single host and
the config section name of the rgw changed from
[client.rgw.$(hostname)] -> [client.rgw.$(hostname).rgwX]
when X is the sequence number: 0,1,2,...
So we should assign 'rgw_zone' item to the exact rgw instance
config section in ceph.conf

Signed-off-by: guihecheng <guihecheng@cmiot.chinamobile.com>