bagpipe-bgp: exit 0 on shutdown
bagpipe-bgp: exit 0 on shutdown

raising SystemExit results in exiting with error code 1
which seems to make systemd complain

with this change sys.exit(0) is used instead, and gives
a proper exit code

Change-Id: I79999beb50cfe564a4536c5963eb2580baea602b

openstack/networking-bagpipe,openstack/networking-bagpipe,stackforge/networking-bagpipe-l2,openstack/networking-bagpipe-l2,stackforge/networking-bagpipe-l2,openstack/networking-bagpipe-l2