bgp: local preference support

add local preference support in bgp. It is possible to apply local
preference to specific paths by using AttributeMap.

Unfortunately this patch supports only IPv4 path and I'm going to
start writing patches for other route families after this.

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