ceph_test_rados: wait for commit, not ack

First, this is what we wanted in the first place

Second, if we wait for ACK, we may look at a user_version value that is
not stable.

Fixes: #7705
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit f2124c5846f1e9cb44e66eb2e957b8c7df3e19f4)

Conflicts:

	src/test/osd/RadosModel.h