Support VLAN tunnel mapping of VLAN aware bridges
example:
```
set interfaces l2tpv3 l2tpeth0 tunnel-id 2000
set interfaces l2tpv3 l2tpeth0 peer-tunnel-id 2000
set interfaces l2tpv3 l2tpeth0 session-id 2000
set interfaces l2tpv3 l2tpeth0 peer-session-id 2000
set interfaces l2tpv3 l2tpeth0 source-address <local address>
set interfaces l2tpv3 l2tpeth0 remote <remote address>
commit
set interfaces bridge br1 enable-vlan
set interfaces bridge br1 vif 20
set interfaces bridge br1 member interface l2tpeth0 native-vlan 20
set interfaces bridge br1 member interface l2tpeth0 vlan-tunnel 2000
commit
```