VPF FeatureType Cleanup GEOT-2347

The following pull request is based on doing the bare minimum to get the VPF module to respect the new geotools feature model (i.e. GeoTools AttributeType / AttributeDescriptor).

More work is needed to get VPF functional, an evaluation is provided here: https://github.com/jodygarnett/geotools/blob/vpf_cleanup/modules/unsupported/vpf/README.rst

Some code is from the pre-2.0 DataSource "singleton" approach, and may not of completed the migration to the DataStore "factory" subsequently adopted.