Add BGP VPN Interconnection Service

The service aims at supporting inter-connection between L3VPNs
and Neutron resources, i.e. Networks, Routers and Ports.
The framework is generic to also support E-VPN [RFC7432],
which inherits the same protocol architecture as BGP/MPLS IP VPNs.

Related-Prod: PROD-17612
Change-Id: I236016669160f5bc6b52d5563b6471be820ec271
11 files changed