[ClusterLabs] Fence node when network interface goes down

[S Rogers]

Using on-fail=fence is what I initially tried, but it doesn't work 
unfortunately.

It looks like this is because the ethmonitor monitor operation won't 
actually fail when it detects a downed interface. It'll only fail if it 
is unable to update the CIB, as per this comment: 
https://github.com/ClusterLabs/resource-agents/blob/4824a7a83765a0596b7d9856d00102f53c8ce123/heartbeat/ethmonitor#L518