add support for single vxlan device (bridge-vlan-vni-map)

add support for single vxlan device (bridge-vlan-vni-map)

new attribute:

"bridge-vlan-vni-map": {
    "help": "Single vxlan support",
    "example": "bridge-vlan-vni-map 1000-1001=1000-1001",
}

example of config:

auto bridge
iface bridge
      bridge-vlan-aware yes
      bridge-ports vxlan0 swp1
      bridge-stp on
      bridge-vids 1000-1001
      bridge-pvid 1

auto vxlan0
iface vxlan0
      vxlan-local-tunnelip 27.0.0.9
      bridge-learning off
      # vlan 1000-1001 maps to vni 1000-1001
      bridge-vlan-vni-map 1000-1001=1000-1001

Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>