rgw: s/idle_timeout/default_idle_timeout/

Signed-off-by: Loic Dachary <loic@dachary.org>
(cherry picked from da00662191e9ec173be50b551253f4460f369f9d)
Merge pull request #92 from ceph/wip-9126-dumpling

rgw: s/idle_timeout/default_idle_timeout/