Add cpu_l3_cache to polling yaml

We need this for cache monitoring features to work out of the box.

Change-Id: I1d145a7e869416e6c0fe0133d94d126f740db016

etc/ceilometer/polling.yaml

@@ -4,6 +4,7 @@ sources:
       interval: 300
       meters:
         - cpu
+        - cpu_l3_cache
         - memory.usage
         - network.incoming.bytes
         - network.incoming.packets