Update documentation files for VPNaaS

The README file was not updated during the services split to accurately
reflect what project this was. Also, update CONTRIBUTING.rst, HACKING.rst,
and TESTING.rst to point to their Neutron counterparts. This makes
maintaining a single copy easier.

Change-Id: I4d9cee887753fc6d7b8b1116deebd2a90faa0f3b