Use percent metrics for majority-down

Change-Id: Iecaffb76b0c43d848c715cefff075cc646b3405c
diff --git a/nova/meta/heka.yml b/nova/meta/heka.yml
index 214ab54..a793c06 100644
--- a/nova/meta/heka.yml
+++ b/nova/meta/heka.yml
@@ -152,7 +152,7 @@
       description: 'Majority of Nova {{ nova_service }}s are down'
       severity: critical
       rules:
-      - metric: openstack_nova_services
+      - metric: openstack_nova_services_percent
         field:
           service: {{ nova_service }}
           state: up