WIP - Service Insertion - changes required for VPNaaS

TODO_LIST:
1. Implement the new abstract methods for VPNaaS (ServicePluginBase).
These are  @ line 242 @
https://review.openstack.org/#/c/93128/13/specs/juno/service-base-and-insertion.rst

These will be implemented in:
-->(???) file:///home/m/Documents/WORK/NewEclipsey/Github/Neutron/neutron/neutron/extensions/vpnaas.py
-->(???) file:///home/m/Documents/WORK/NewEclipsey/Github/Neutron/neutron/db/vpn/vpn_db.py

2. Change DB models (overlap with above for the db methods?) as explained
in line 307 @
https://review.openstack.org/#/c/93128/13/specs/juno/service-base-and-insertion.rst

3. DB Migrations.