lib,zebra,bgpd,staticd: use label code to store VNI info

lib,zebra,bgpd,staticd: use label code to store VNI info

Use the already existing mpls label code to store VNI
info for vxlan. VNI's are defined as labels just like mpls,
we should be using the same code for both.

This patch is the first part of that. Next we will need to
abstract the label code to not be so mpls specific. Currently
in this, we are just treating VXLAN as a label type and storing
it that way.

Signed-off-by: Stephen Worley <sworley@nvidia.com>