rate_of_change should not calculate delta on delta type sample

[ZhiQiang Fan]

When I read the master branch code, I find that in https://github.com/openstack/ceilometer/blob/master/ceilometer/transformer/conversions.py#L129

 volume_delta = (s.volume - prev_volume
                               if (prev_volume <= s.volume or
                                    s.type != sample.TYPE_CUMULATIVE)
                               else s.volume)

which means if a sample is DELTA type, and we apply rate of change on this meter, then the result can be wrong.

However, in current default pipleline, there is no such use case, so this is not found in CI or user test. Since all DELTA type meters come from notifications, I think this might not be triggered in most user cases too. But storage.objects.{incoming,outgoing}.bytes can be exception, and we would never know what third party developers will do, so maybe we should better fix it.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/123320

[OpenStack Infra (hudson-openstack)]

Change abandoned by ZhiQiang Fan (<email address hidden>) on branch: master
Review: https://review.openstack.org/123320
Reason: let cloud operator configure it carefully