qa/tasks: use new reliable flush_pg_stats helper

qa/tasks: use new reliable flush_pg_stats helper

The helper gets a sequence number from the osd (or osds), and then
polls the mon until that seq is reflected there.

This is overkill in some cases, since many tests only require that the
stats be reflected on the mgr (not the mon), but waiting for it to also
reach the mon is sufficient!

Signed-off-by: Sage Weil <sage@redhat.com>