mgd/ddm should advertise a lower tunnel exit metric when all BFD links are down

[rcgoodfellow]

In #174 we implemented mechanisms for a graceful transition to and from BFD networks. This could be potentially problematic when we have one switch that has active BFD and another switch that has no active BFD sessions but active RIB entries associated with down BFD sessions. Both switches will advertise as equals to the underlay as tunnel exit points. In this situation, the switch with no active BFD sessions should advertise with a different tunnel origin metric (but obviously only if it's been configured with BFD). This way OPTE endpoints sending traffic to tunnel exit points can prefer the switch with active BFD sessions.