Measurements documentation update

Nova measurements:
- add cpu_util
- del disk.io.requests, disk.io.bytes
- add instance unit to instance:*

+fix some errors on type and unit

for all measurements add new column 'Origin' :
notification|pollster|both

tag : DocImpact
Fixes: bug #1164032

Change-Id: I0046032b8835168772fc7fff6c34a9be99ddbd56
Merge "Measurements documentation update"