bgp: fix the way of getting RouteFamily object

RouteFamily objects are treated as a singleton object.
However they are created in peer.py, so I added method to
get RouteFamily object in bgp.py and changed codes to use it.

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