Fix polling namespace on compute nodes

Previously only ceilometer-polling services running in the central
container had a polling_namespace configured. Those running on compute
nodes used the default which is to run all pollsters from the central
and compute namespace. But there is no need to gather the data for the
central namespace on every compute node.

With this fix the central container only runs the central pollsters and
those on the compute nodes only the compute pollsters. This can be
adjusted by config_template overrides in group_vars if needed.

Change-Id: Ia130bd7f9000882b556aea2190de97ddb290bafc

releasenotes/notes/configure-polling-namespaces-226573c37a44b5a6.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    ceilometer-polling services running on compute nodes did not have the
+    polling namespace configured. Because of this they used the default value of
+    running all pollsters from the central and compute namespaces. But the
+    pollsters from the central namespace don't have to run on every compute
+    node. This is fixed by only running the compute pollsters on compute nodes.

templates/ceilometer.conf.j2

@@ -28,6 +28,8 @@ event_dispatchers = database
 
 {% if 'ceilometer_agent_central' in group_names %}
 polling_namespaces = central
+{% elif 'ceilometer_agent_compute' in group_names %}
+polling_namespaces = compute
 {% endif %}
 
 [oslo_messaging_rabbit]