bgpd: add define of compilation for vpn commands

bgpd: add define of compilation for vpn commands

Use KEEP_OLD_VPN_COMMANDS define to brace some vty commands used by bgp
to configure or show vpnvx contexts using old method, that is to say
using vty keyword vpnv4 or vpnv6 command.
In addition to this, the commit adds two new commands under route-map
with new format:
[no] set ipv4|ipv6 vpn nexthop <IPv4 Address> | <IPv6 Address>
It also add following command in old format:
[no] set vpnv6 nexthop <IPv6 Address>
Note that the commit does not take into account the availability of old
commands that are not available in new format.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com