rgw: list container only shows stats if needed

Fixes: #4759
Add a new request param 'stats' for the swift list containers
request. If set to 'false' it disables stats retrieval, which
makes it go faster. Also, don't dump stats if format is plain,
as they're not going to be dumped.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>