Merge pull request #6709 from xiaoxichen/failure_by_host

mon: support min_down_reporter by subtree level (default by host)

Reviewed-by: Kefu Chai <kchai@redhat.com>
Reviewed-by: Sage Weil <sage@redhat.com>

diff --cc src/common/config_opts.h
Simple merge

diff --cc src/mon/OSDMonitor.cc
Simple merge