IPv6 and SDN-IP: Verify the receiving of IPv6 routes over IPv6 BGP peering

Currently, the BGP implementation as part of SDN-IP implements
the receiving of IPv6 routes (by using the BGP Multiprotocol Extensions).
Such routes could be received over either IPv4 or IPv6 BGP peering
(with the appropriate configuration on the BGP speaker side).
So far, we have tested the receiving of IPv6 routes only over IPv4 peering.

We (should) have all appropriate support for the configuration of IPv6 BGP peers in ONOS (i.e., IPv6 point-to-point intents), and we should be able
to use such peers to receive IPv6 routes.

This sub-task is to verify whether we the IPv6 peering configuration actually
works, and whether the IPv6 peers can be used to receive IPv6 routes.