tasks/rgw: fix config parsing for idle timeout

This was broken in 54a7298c.  The configs had a
'default_idle_timeout' setting added, but the code
was looking for 'idle_timeout', so leaving the
'default_idle_timeout' items in the config dict
where they later broke the parsing of per-client
options.

Fixes: #9126

Signed-off-by: John Spray <john.spray@redhat.com>