Fix ability to add custom volume_backend_name

This patch fixes the ability to set the volume_backend_name
for each driver.   The original bug was filed against LeftHand
and 3PAR, but several drivers exhibited a fixed backend name.

Fixes: bug #1175222
       bug #1172286

Change-Id: I4197f99bebd6ca8e4969795ae6edb537bc066c90
Merge "Fix ability to add custom volume_backend_name"