rgw: set placement targets for zones
rgw: set placement targets for zones

This lets zones use different pools for actual object data.

Signed-off-by: Josh Durgin <51652c783f70f5b272ebd02f1fd20d3af7a2449f@inktank.com>

ivotron/teuthology,ktdreyer/teuthology,zhouyuan/teuthology,yghannam/teuthology,dmick/teuthology,tchaikov/teuthology,caibo2014/teuthology,SUSE/teuthology,tchaikov/teuthology,robbat2/teuthology,ceph/teuthology,michaelsevilla/teuthology,ceph/teuthology,dmick/teuthology,SUSE/teuthology,ivotron/teuthology,dreamhost/teuthology,michaelsevilla/teuthology,SUSE/teuthology,robbat2/teuthology,zhouyuan/teuthology,t-miyamae/teuthology,yghannam/teuthology,caibo2014/teuthology,t-miyamae/teuthology,dmick/teuthology,dreamhost/teuthology,ktdreyer/teuthology