BUG-338 Allow incomming BGP connections.

Added BGPPeerRegistry that holds all configured BGP peers.
Client/Server SessionNegotiators retrieve BGPPeers(SessionListeners) from the BGPPeerRegistry.
Added StrictBGPPeerRegistry (default implementation of BGPPeerRegistry) that allows only 1 session with BGP peer. If second session is being established, the registry drops either already established or new BGP session (depending on higher source BGP ID).

Added 2 config attributes to BGPPeer:
- initiate connection: true to start BGP connection from ODL. False if just configure BGP peer to accept session from remote device.
- peer registry: dependency on BGPPeerRegistry. Peer registers itself into this registry.

* new attributes are optional to preserve backwards compatibility.

Change-Id: I2f7a606db5196fed49094d49efbb570c4d0fbecb
Signed-off-by: Maros Marsalek <mmarsale@cisco.com>