Initializing server:publisher dict in ceilometer

The patch initializes ceilometer:server:publisher dict to
empty to be overridden further.

Change-Id: Ic0ba497225f34564b9adb499579c70273aea13ff
Related-PROD: PROD-18827
diff --git a/ceilometer/server/cluster.yml b/ceilometer/server/cluster.yml
index 11488fb..6f8d3f8 100644
--- a/ceilometer/server/cluster.yml
+++ b/ceilometer/server/cluster.yml
@@ -13,7 +13,7 @@
       secret: ${_param:ceilometer_secret_key}
       ttl: 86400
       publisher:
-        default:
+        default: {}
       bind:
         host: ${_param:cluster_local_address}
         port: 8777