bgp: add some constatns

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
config: add helper function to set defalut values

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
config: add helper functions to handle config file

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>