mon: OSDMonitor: HEALTH_WARN on 'mon osd down out interval == 0'

Having this option set will only be reported as a HEALTH_WARN if it
is set on the leader.  Only the leader will report this as well, as
the peons will have no way to know whether the leader has this option
set.

Fixes: 7784

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>