qa: ignore normal slow IO warnings for cephfs

qa: ignore normal slow IO warnings for cephfs

Generally the slow warnings we get are just over the threshold. These warnings
are related to deploying multiple Ceph daemons side-by-side. Let's see how we
do with two minutes.

Ignoring the warnings entirely is unsatisfactory as they serve as a useful
canary in the coal mine when you see warnings for ops > some unreasonably large
amount of time.

Fixes: http://tracker.ceph.com/issues/26900
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit fc6e683832125a1042638f21b623e29d0d4c34d8)