bgp: add parameter for site_of_origins to neighbor_add and vrf_add method

Fujita-san,
thank you for your comment and finding the bug that causes test case failure.
I fixed the second and resend the whole patch.

The first is the addition of site_of_origins parameter to neighbor_add and
vrf_add method and fixes to some import error.

Signed-off-by: Hiroshi Yokoi <yokoi.hiroshi@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
bgp: add code to handle RD 0:0 at the head of nexthop network address in MP_REACH_NLRI path attribute

The second is patch to add or skip RD 0:0 label for the next_hop address in MP_REACH_NLRI attribute.

According to RFC, next_hop address seems to be required to contain RD of 0 when advertise VPN prefix.
This is defined in RFC 4659(3.2.1.1.  BGP Speaker Requesting IPv6 Transport) for VPN-IPv6
and in RFC 4364(4.3.2.  Route Distribution Among PEs by BGP) for VPN-IPv4.

Signed-off-by: Hiroshi Yokoi <yokoi.hiroshi@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
bgp: fix conflict of PEER_NEXT_HOP and NEXT_HOP value

The third is fix to a conflict of PEER_NEXT_HOP value in neighbors.py.

There is a conflict between PEER_NEXT_HOP in neighbors.py and NEXT_HOP in api/base.py.
As a result of the conflict, the correct validator for PEER_NEXT_HOP is not used when
neighbir_add is executed.

Signed-off-by: Hiroshi Yokoi <yokoi.hiroshi@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>