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

A 'status' or 'health' request will return a HEALTH_WARN whenever the
monitor handling the request has the option set to zero.

Fixes: 7784

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
(cherry picked from commit b2112d5087b449d3b019678cb266ff6fa897897e)