Allow short names for volume_driver in cinder.conf

This allows configuring the volume driver like:
volume_driver="nfs"
instead of
volume_driver="cinder.volume.drivers.nfs.NfsDriver"

The setting is also treated as case-insensitive to allow "LVM" or
"lvm", etc.

Driver identifiers not listed in the translation table will be
treated as class paths, as before.

Change-Id: I9e28e2afb07c88353a546a87e76a3220b051c9f2