mgr/devicehealth: add helpers to life_expectancy_response()

mgr/devicehealth: add helpers to life_expectancy_response()

- if mark_out_threshold is met we write to log.warn instead of raising a
  health warning.
- check that OSD is 'in' before calling mark_out().
- raise a health warning in case OSD is marked 'out' but still has PGs
  attached to it.
- cast thresholds default values to string.
- add SCSI multipath support to health warning message.
- change health warning message.

Signed-off-by: Yaarit Hatuka <yaarithatuka@gmail.com>