Change default polling interval

Gnocchi by default uses the low archive policy. Aodh alarm examples
and heat template examples use 300 as granularity as well.

But ceilometer generates sample every 600 seconds making
alarming/autoscaling doesn't work out of the box.

People have to lower ceilometer polling or raise Gnocchi/Aodh/Heat
granularity to make thing works.

Change-Id: I3f234e208f3389d6e5952782aa9b310594bfaa5f

etc/ceilometer/polling.yaml

@@ -1,6 +1,6 @@
 ---
 sources:
     - name: all_pollsters
-      interval: 600
+      interval: 300
       meters:
           - "*"