qa/workunits/rados/test_health_warning: misc fixes

qa/workunits/rados/test_health_warning: misc fixes

* do not let osd shutdown itself by enlarge osd_max_markdown_count and
  shorten osd_max_markdown_period
* do not shutdown all osds in the last test. if all osds are shutdown at
  the same time. none of them will get updated osdmap after noup is
  unset. we should leave at least one of them, so the gossip protocol
  can kick in, and populate the news to all osds.

Fixes: http://tracker.ceph.com/issues/20174
Signed-off-by: Kefu Chai <kchai@redhat.com>