[2.1] Neighbour monitoring emits MOVED events for bogus MACs

[Mike Pontillo]

Steps to reproduce: plug the IPMI port on a SuperMicro board into a different switch port. (a different switch port may not be required: link down, then link up may be sufficient.)

Observed the following event in neighbour discovery:

{"event": "MOVED", "mac": "00:00:00:00:00:00", "vid": null, "time": 1470184832, "previous_mac": "0c:c4:7a:c0:2e:b4", "ip": "172.16.42.150"}
{"event": "MOVED", "mac": "0c:c4:7a:c0:2e:b4", "vid": null, "time": 1470184832, "previous_mac": "00:00:00:00:00:00", "ip": "172.16.42.150"}

We are emitting a MOVED event for an invalid MAC address ("00:00:00:00:00:00") which should be ignored.

It immediately moves back to the original MAC, but that part is okay. (it should be REFRESHED instead when this happens.)