addons: vxlan: add support for vni filter on single vxlan device

addons: vxlan: add support for vni filter on single vxlan device

- create single vxlan device with vnifilter flag
- install vni filter with vnis from bridge-vxlan-vni-map
- vni filter can only be applied when the vxlan interface
is in down state
- toggling of vni filter is unsupported (maybe in the future)
- vni filter on a single vxlan or collect metadata/external
device is a new kernel feature yet to be upstreamed
- move vlan/vni id math helpers to utils.py

Reviewed-by: Julien Fortin <jfortin@nvidia.com>
Signed-off-by: Roopa Prabhu <roopa@nvidia.com>
Signed-off-by: Julien Fortin <jfortin@nvidia.com>