Comment 1 for bug 1607596

In the plan, a bgpvpn will have multiple techniques, it conflict with bgpvpn typ, because there is only 1 bgpvpn type.
There is 3 options:
1. bgpvpn will have ONLY 1 technique.
   In some case, we want a VPN can support L2 EVPN and l3 evpn-prefix. This case can not be met.

2. Remove bgpvpn_type from bgpvpn table, because technique pairs include the type.

3. Add a new bgpvpn type: 'evpn', it support both evpn-prefix and RFC7432.

For me, I prefer to #3.