Make sure to select valid, non-reserved, non-used table ids

[Fredi-raspall]

When creating VRF kernel interfaces, we need to make sure that we don't select table-ids that are in use by other VRF interfaces or by the default VRF / kernel (e.g. reserved ids for local, main or default).

Currently, we "grant" uniqueness by setting the table-id equal to the VNI for a VRF.
This is "fine" since we ensure that VNIs are unique, but VNIs equal to 253, 254 or 255 may be problematic and lead to failures.

Task: make sure that we select table ids that are unique and non-reserved.