bgp: fix bug in net_ctrl listen_tcp

due to fdce89a0, _listen_tcp returns tuple.

Reported-by: 坪井俊樹 <t.tsubo2000@gmail.com>
Signed-off-by: ISHIDA Wataru <ishida.wataru@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
bgp: shut up runtime warning due to configuration file loading

this patch shut up the warning shown below

/home/wataru/etc/bgpconf01.py:1: RuntimeWarning: Parent module
'bgpspeaker' not found while handling absolute import
  import os

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