Work toward IPv6 support in BGP: implement decoding/encoding of the
BGP OPEN Multiprotocol Extensions Capabilities (RFC 4760).

The corresponding BGP UPDATE decoding is not done yet, hence for now
we don't include any Capabilities in the BGP OPEN message originated by us.

This work is in the context of ONOS-422.

Change-Id: I8e1c8838adc189aa32a8edf98be976d90fc4ad42