Support VLAN tunnel mapping of VLAN aware bridges
https://legacy.netdevconf.info/2.2/slides/prabhu-linuxbridge-tutorial.pdf
example:
```
set interfaces vxlan vxlan0 external
set interfaces vxlan vxlan0 source-address <source>
set interfaces vxlan vxlan0 mtu 1500
commit
set interfaces bridge br1 enable-vlan
set interfaces bridge br1 vif 20
set interfaces bridge br1 member interface eth0 vlan-tunnel 2000 vlan 20
set interfaces bridge br1 member interface eth0 vlan-tunnel 2000 remote <remote>
commit
```