qa: wait longer for osd to flush pg stats

qa: wait longer for osd to flush pg stats

pg sends pg-stats to mgr every 5 seconds, so we cannot check for the
number of pgs right after creating the pool, at that moment, the number
of pgs could be 0, that's why manger.wait_for_clean() returns right
away, and leaves us with 0 pgs: the pgs serving the pool are still being
created. that's why `manager.get_num_active_clean()` returns `0`
sometimes. so, we should force osd to flush their stats to mgr, and wait
until the pg stats converages.

Fixes: http://tracker.ceph.com/issues/24321
Signed-off-by: Kefu Chai <kchai@redhat.com>