rgw: increase idle_timeout

It appears to be necessary to bump the idle timeout when tests are run
on VPS which are slower than bare meta. The proposed 1200 may be
overkill but there is no downside to having a large timeout. The only
problem would be to not have any timeout and a never ending job.

http://tracker.ceph.com/issues/8988 Fixes: #8988

Signed-off-by: Loic Dachary <loic@dachary.org>