Change vni through NicUpdate

[jerome-jutteau]

We need to be able to move a (virtual) nic to another vxlan.
User could update a nic vni through NicUpdate.
If vni change, we could make a down/up on interface to trigger a DHCP request.

If we don't have this feature, we could not move a nic to a different network :)

EDIT: still to be discussed