[Discovery] UnicastZenPing should also ping last known discoNodes

At the moment, when a node looses connection to the master (due to a partition or the master was stopped), we ping the unicast hosts in order to discover other nodes and elect a new master or get of another master than has been elected in the mean time. This can go wrong if all unicast targets are on the same side of a minority partition and therefore will never rejoin once the partition is healed.

Note: this is agains the `feature/improve_zen` branch
Wait this code is ok, we want nodes with master eligible nodes to be in the beginning of the list and therefor it should be deemed smaller (return `-1`) then a node with that isn't master.

Maybe add a unit test for the elect master logic :)