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.  The leader
will furthermore inform on 'health detail' that in such case it's the
same as having the 'noout' flag set.

Fixes: 7784

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>