bgp: show VPNv4 Prefix information

But the result looks unexpected regarding of show_command .
I can’t find the advertised labels or assigned labels for vpnv4 prefix .
 (reference: http://sourceforge.net/p/ryu/mailman/message/32686423/ )
Therfore, I've patched the codes for formatting as follows .
=> needs for fixing labels attribute after that .

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

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