ceph: more detailed status output for nagios check

Don't return 2 (critical) on HEALTH_WARN. Instead, use proper all
possible nagios plugin return codes: map HEALTH_WARN to WARNING,
HEALTH_ERR to CRITICAL and HEALTH_OK to OK (duh). If none of these are
detected, fall back to UNKNOWN.

Change-Id: I40a2d01e0f4f6905d1f2b296d4d9520251c092cf