Fix alarm definition for cinder-volume

This commit fixes an alarm definition for cinder-volume where "service"
was set to "scheduler" instead of "volume".

With this change the "cinder-volume" and "cinder-data" clusters are now
correctly reported as UNKNOWN in Grafana and Nagios.

Change-Id: I7001fa6d5c4923d455bff90a8863b1ff05851be2
diff --git a/cinder/meta/heka.yml b/cinder/meta/heka.yml
index e827440..51beb04 100644
--- a/cinder/meta/heka.yml
+++ b/cinder/meta/heka.yml
@@ -133,7 +133,7 @@
       rules:
       - metric: openstack_cinder_services
         field:
-          service: scheduler
+          service: volume
           state: down
         relational_operator: '>'
         threshold: 0