bgp: fix bug of labels formatting for show VPNv4 prefix information

The field of 'Labels' has not displayed in properly because of extracting 'label_list' in python slicing tuples.

INFO:bgpspeaker.api.base:API method operator.show called with args: {'params': ['rib', 'all'], 'format': 'cli'}
Status codes: * valid, > best
    Network                          Labels   Next Hop             Reason          Metric LocPrf Path/Origin
Family: rtfilter
*>  64512:64511:101                  None     0.0.0.0              Only Path                     2
Family: vpnv6
Family: vpnv4
*>  64511:101:10.10.0.1/32           ([17],)  192.168.100.100      Only Path       0             64511 2
*>  64511:101:10.20.2.0/24           ([100],) 0.0.0.0              Only Path                     2
*>  64511:101:10.20.1.0/24           ([100],) 0.0.0.0              Only Path                     2
*>  64511:101:10.20.3.0/24           ([100],) 0.0.0.0              Only Path                     2
Family: ipv4
Family: ipv6

Reported-by: Toshiki Tsuboi <t.tsubo2000@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
bgp: add route server function to bgpspeaker

implemented draft-ietf-idr-ix-bgp-route-server-05 partially.

you can enable this function like below

>    speaker = BGPSpeaker(as_number=64512, router_id='10.10.0.1')
>    speaker.neighbor_add('10.0.0.1', 64514, is_route_server_client=True)
>    speaker.neighbor_add('10.10.0.2', 64513, is_route_server_client=True)

add multiple rib function is a future work

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