qa/workunits/ceph-helpers: test wait_for_health_ok differently
0 OSDs is not an error anymore in the new health checking implemented by
OSDMap::check_health(). this case was treated as an error before, see
OSDMonitor::get_health(). but an osdmap without any OSD is fine, i
think. but an osdmap with 3 OSDs, but all of them are down and out, this
is an error. and we do report this as an error. so, let's update the
test instead.