Add internal option [DEFAULT]shared_storage to ocata

Shared storage for instances or not.

This option is used to distinguish different way to calculate disk usage
while getting hypervisor-stats.

ENABLE THIS ONLY WHEN ALL COMPUTE NODES ARE USING THE SAME SHARED STORAGE!
Otherwise stats report will be skewed.

Possible values:

* True: Average all the compute node's disk usage
* False: Sum all the compute node's disk usage

Change-Id: Ia9645cb16ea21dc62bb9ff664911c99beeeb01f4
Related-Prod: PROD-25240
2 files changed