ceph: change CephAbsentMgr to use 'up' query

Instead of using 'absent' query, we are trying to use 'up' which should
provide us with the needed 'namespace' field in the resultant metrics

Signed-off-by: aruniiird <amohan@redhat.com>

cluster/examples/kubernetes/ceph/monitoring/prometheus-ceph-v14-rules.yaml

@@ -42,7 +42,7 @@ spec:
         severity_level: critical
         storage_type: ceph
       expr: |
-        absent(up{job="rook-ceph-mgr"} == 1)
+        up{job="rook-ceph-mgr"} == 0
       for: 5m
       labels:
         severity: critical