tests: resolve ceph-helpers races

Some tests were racing against the monitor. On a fast machine it worked
but slower machines (or sometime when running in parallel), the monitor
is lagging behind. Use wait_for_clean to make sure the monitor is in the
desired state for the test to succeed.

http://tracker.ceph.com/issues/10384 Fixes: #10384

Signed-off-by: Loic Dachary <ldachary@redhat.com>