Add cinder metrics to polling.yaml

As of [1] you can skip using cinder-volume-usage-audit
and instead let the ceilometer-polling agent poll the
cinder API for the data.

Ceilometer itself should know if cinder is available
or not.

[1] https://review.openstack.org/#/c/388584/

Change-Id: I1bd6e45060f90fcbc44658f75af45e1b4ebc7acf

templates/polling.yaml.erb

@@ -14,6 +14,9 @@ sources:
         - disk.read.requests
         - disk.write.bytes
         - disk.write.requests
+        - volume.size
+        - volume.snapshot.size
+        - volume.backup.size
         - hardware.cpu.util
         - hardware.memory.used
         - hardware.memory.total