rgw: change json formatting for swift list container

Fixes: #4048
There is some difference in the way swift formats the
xml output and the json output for list container. In
xml the entity is named 'name' and in json it is named
'subdir'.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 3e4d79fe42dfc3ca70dc4d5d2aff5223f62eb34b)