Storwize: Fix iogrp availability check

Do not check if vdisk_count is greater than zero to determine iogrp
availability. This is wrong, and leads to the driver not being able to
use empty iogrps.

Change-Id: Ia73189a11cfcc0641c1527293675361c4e78443f
Closes-Bug: #1240395
Nexenta NFS driver: caching for appliance volroot

Added caching for NexentaStor appliance 'volroot' value.

Change-Id: I3803981cc8c9b6c47dc9fccc6b2414e21d9d6285
Merge "Nexenta NFS driver: caching for appliance volroot"
Merge "Storwize: Fix iogrp availability check"