Add support for VXLAN

Pull request for CLI extension for kernel VXLAN interface.
Base codes are https://github.com/upa/vxlan-vyos .

For examples:
set interfaces vxlan vxlan0 vni 0
set interfaces vxlan vxlan 0 group 239.0.0.1
set interfaces vxlan vxlan0 address 10.0.0.1/24
or
set interfaces vxlan vxlan0 bridge-group bridge br0