l2tp-remote acces authentication require options.

Hi daniil, I added require autentication  options to ppp/options.xl2tp and force aut protocol to pap|chap|mschap|mschap-v2 (don't added eap because I don't tested it) 
Also, options.pptp don't need this because has has a require mschap-v2
The shortest way should be add directly require mschap-v2 to options.xl2tp, but to mantain compatibility with previous installations I created this mod.

Let me know if I you prefer that auth options in xl2tp would be like auth options.pptp
Skeleton was:
vpn {
    l2tp {
        remote-access {
            authentication {
                mode radius
                radius-server 10.15.8.2 {
                    key ****************
                }
                  require mschap-v2 <-------chap,pap,....