Redesigning VNI allocation

[ipspace]

With the benefits of perfect hindsight: we never should have allocated VNI values in the VLAN module. It seemed like a good idea at that time, but turned out to be less-than-ideal now that I'm considering EPVN-over-MPLS.

So here's the new idea (that I would introduce in 1.4 -- see #510):

• VLAN module allocates vlan.id values
• VXLAN module uses the list of VXLAN-enabled VLAN modules to allocate vlan.vni values
• The default list of VXLAN modules (when missing) is "all VLANs", replicating the existing behavior with vlan.auto_vni set to True
• Using an explicit list of VXLAN-enabled VLANs is equivalent to vlan.auto_vni set to False
• Static vlan.vni values (integer or boolean) will still be supported
• At the end of the transformation, the node vxlan.vlans list will reflect the VXLAN-enabled VLANs (those with valid VNI values)

Any objections or suggestions @ssasso @jbemmel?

[jbemmel]

See #317 - I've always felt that the vxlan module would be a better place for VNIs

[ipspace]

And you were right ;)

What about the auto_vni. Are you OK with dropping it and replacing it with an explicit list of VLANs?