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
(cherry picked from commit 781055bc1f30cebe08848130f3a40764f7f1bb4f)
Dont delete backup record from database

Instead we should be marking the backup as deleted so that we have an audit
of all the backups. Backups are now marked deleted in the way that volumes
are.

Fixes bug: 1169943
Fixes bug: 1169857

Change-Id: I52c603be138c0f1d6d411d167977041255ac9053
(cherry picked from commit a4ba17b668bd893d50fd4f3a5453da89966a77e3)
Merge "Add stats reporting to Nexenta Driver" into stable/grizzly
Merge "Dont delete backup record from database" into stable/grizzly