InterVpnLink: push_mpls flows not installed in FIB when there are no VMs in VPNs

InterVpnLink is a way of sharing certain routes between 2 different external VPNs. It works more or less like this: BGP router is instructed to send packets belonging to VPN A towards a designated OF switch. The switch switches from VPN A to VPN B and sends the packet back to the BGP router where VPN B's VRF is used.

The flows in FIB table that sends the packet back to the BGP router are not being installed if VPN B has no footprint at all, that is, there is no VM belonging to VPN B connected to any OF Switch. As soon as there exists one VM in VPN Footprint everything works as expected. However, the case when there is no VM in VPN Footprint must be covered as well.