bgp: supporting loopback interface as neighbour_source_address for iBGP

In generally, iBGP session is established between loopback interfaces.
Therefore, we need to specify loopback interface as neighbour_source_address.
This parameter is just like update-source command in cisco router.

Signed-off-by: Toshiki Tsuboi <t.tsubo2000@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>