bgpd: Add constants for some repetitive CLI strings

bgpd: Add constants for some repetitive CLI strings

"Address Family\n"
"Address Family modifier\n"

Before:

```
donatas-laptop(config-router)# address-family ipv4
  <cr>
  flowspec         Address Family Modifier
  labeled-unicast  Address Family modifier
  multicast        Address Family modifier
  unicast          Address Family Modifier
  vpn              Address Family modifier
```

After:

```
donatas-laptop(config-router)# address-family
  ipv4   Address Family
  ipv6   Address Family
  l2vpn  Address Family
donatas-laptop(config-router)# address-family ipv4
  <cr>
  flowspec         Address Family modifier
  labeled-unicast  Address Family modifier
  multicast        Address Family modifier
  unicast          Address Family modifier
  vpn              Address Family modifier
```

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>