Please can you add support for unencrypted L2TPv2 (RFC2661) clients to VyOS.
Configuration might look something like this:
```
set interface l2tpv2 l2tp0 server l2tp.aa.net.uk
set interface l2tpv2 l2tp0 default-route force
set interface l2tpv2 l2tp0 mtu 1492
set interface l2tpv2 l2tp0 enable-ipv6
set interface l2tpv2 l2tp0 user-id <Username>
set interface l2tpv2 l2tp0 password <Password>
```
Andrews and Arnold (AAISP) offer this as a commercial service for people using 'inferior broadband':
https://aa.net.uk/broadband-l2tp.html
They provide configuration guides:
* https://support.aa.net.uk/L2TP_Client:_Linux (using xl2tpd and pppd)
* https://support.aa.net.uk/L2TP_Client:_OpenWRT
* https://support.aa.net.uk/L2TP_Client:_Cisco
The Cisco configuration seems the least intuitive.
As originally asked here:
https://forum.vyos.io/t/l2tp-for-ip-tunnel/3166