bgpd: Strip neighbors's description to 20 chars in show bgp summary

bgpd: Strip neighbors's description to 20 chars in show bgp summary

Also make sure we do not show descriptions with whitespace characters, just
take the first splitted by whitespace.

```
root@exit1-debian-9:~/frr# vtysh -c 'show run' | grep description
 neighbor 192.168.0.2 description 12346789012345678901234567890
root@exit1-debian-9:~/frr# vtysh -c 'show ip bgp summary' | grep 192.168.0.2
192.168.0.2     4      65030         0         0        0    0    0    never Idle (Admin)        0 1234678901234567890
```

```
root@exit1-debian-9:~/frr# vtysh -c 'show run' | grep description
 neighbor 192.168.0.2 description one two
root@exit1-debian-9:~/frr# vtysh -c 'show ip bgp summary' | grep 192.168.0.2
192.168.0.2     4      65030         0         0        0    0    0    never Idle (Admin)        0 one
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>