Add stats reporting to Nexenta Driver

Nexenta driver was never updated to report stats.
This change doesn't include support for backend_name assignment
via configs, there's some other work that will need to be
done to enable that.

Fixes bug: 1166607

Change-Id: Id9cfff24df8815a67e42393aa18fd41ac1b9e6b3
Merge "Add stats reporting to Nexenta Driver"