feature: having different vlan pool allocations for UNIs and NNIs

[viniarck]

Currently, available_tags belongs to the interface, but currently, it's supported that an EVC can also be created in a NNI interface, which starts acting as a UNI for that EVC. In this case, since it uses the same pool, link down events for the NNI being used by other EVCs might allocate the UNI s vlan if that ever gets released by the EVC (by changing it or deleting), so changing certain configurations might cause this dispute that can be cumbersome for network operators.

Ultimately, it's desired to have an explicit non overlapping pool for UNIs and NNIs of a given interface. That will be discussed when this gets prioritized, making sure if meets the current usages that well known in production. We're not too far away from this, it's mostly a matter of having a separate pool while still allocating.

Related prior discussions: kytos-ng/kytos#406