Remove RetypeVD class, fix NFS driver retype

Remove RetypeVD class, fix NFS driver retype

retype() only exists in the deprecated VolumeDriver() class
and not BaseVD.  This means that drivers inheriting from *VD
hit attribute errors when trying to call driver.retype().

The thinking here is that RetypeVD isn't really necessary,
since all drivers must implement some version of the retype
method (even if it's a no-op with just a standard return code).

Therefore, just rely on this being in the base driver class(es)
rather than making it a separate abc class.

Closes-Bug: #1478987
Related-Bug: #1471807

Change-Id: Ie0c6f23caf68080a05a10cc7ad245878a057fb02