In case VLAN networks are being used in neutron for providing external
connectivity they will be assinged an IP address as well in order to
get ARP/NDP proxying to work.
With that VRF exposure method was relying on broken behaviour, as it
never needed/expected ARP/NDP to be enabled var a vlan rather then bridge
as well as vlan device to exist at all. So skipping vlan_id for evpn makes
sense in this case, as only offload is important.
Closes-Bug: #2088558
Change-Id: Ibffda84801150a18ee8c441b4396afc900d8bc36
fd029f985d