fix the failure to start cinder-volume for powervc driver

PowerVC cinder driver __init__ does not call it's super's __init__,
and latest cinder driver change result in cinder-volume can not be
started due to this reason.

Closes-Bug: #1363846
Change-Id: I4743513b115fd02521fac546985981ea5197527c
(cherry picked from commit 1e8cddd0a7f861479106d03cabab3d2ba19e1925)