ceph_test_rados_api_tier: do fewer writes in HitSetWrite

We don't need to do quite so many writes.  It can be slow when we are
thrashing and aren't doing anything in parallel.

Fixes: #8932
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2146 from ceph/wip-8932

ceph_test_rados_api_tier: do fewer writes in HitSetWrite

Reviewed-by: Greg Farnum <greg@inktank.com>