Replace the NetApp driver proxy layer with a proper factory.

The proxy implementation required a hack in the Abstract Base
Classes project in Cinder core.  This factory implementation
removes the need for the hack.  It also improves logging, as
the actual driver object is provided to the volume manager
instead of the driver proxy.

Change-Id: I28f4798b9e799547d1696d2f135d3302b3c553fe
Closes-Bug: 1405060
Merge "Replace the NetApp driver proxy layer with a proper factory."