re-add cpu_util metric to measurement list
re-add cpu_util metric to measurement list

- it was wrongly removed by bug 1597621.
- add note to mention you don't necessarily need rate_of_change if
using Gnocchi

Change-Id: I352cfcebca559b5e47502e2818e62854b971e3ff

openstack/ceilometer,openstack/ceilometer