[Helium] Unwanted flow entries are removed when link-down is detected.

VTN Manager in Helium release may remove unwanted flow entries when link-down is detected.

How to reproduce:

1. Start mininet with 3 depth tree topology.

% sudo mn --controller=remote,ip=controller-ip --topo tree,3

2. Create a VTN and a vBridge, and map untagged frames using VLAN mapping.

POST /controller/nb/v2/vtn/default/vtns/vtn_1
Body: {}
POST /controller/nb/v2/vtn/default/vtns/vtn_1/vbridges/bridge_1
Body: {}
POST /controller/nb/v2/vtn/default/vtns/vtn_1/vbridges/bridge_1/vlanmaps
Body: {}

3. Establish unicast flows by sending ICMP echo.

mininet> pingall

4. Shut down the network interface in host h1.

mininet> h1 ifconfig h1-eth0 down

This should uninstall flow entries whose source or destination host is h1.
But VTN Manager will uninstall all the flow entries.