rgw: set data extra pool correctly

When configuring multiregion zone, set the data extra pool

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Merge pull request #291 from ceph/wip-rgw-pools

rgw: set data extra pool correctly

Backport: next, firefly
Reviewed-by: Sage Weil <sage@redhat.com>