bgp: add med parameter to neighbor_add

Sorry, I changed parameter name for MED to uniform with add_vrf.
I resend patches for neighbor_add method and neighbor_update and
please ignore the patches that I sent before.

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

added neighbor_update method to BGPSpeaker class.
Currently only MED value can be changed.

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