rgw: increase the number of OSDs for erasure coded pools

So that there never is less than 4 osd which is the minimum when trying
erasure coded workloads that only work with 4 osd.

http://tracker.ceph.com/issues/9298 Fixes: #9298

Signed-off-by: Loic Dachary <loic-201408@dachary.org>