Default to localhost if zookeeper has no servers set (#3056)

plugins/inputs/zookeeper/zookeeper.go

@@ -43,7 +43,7 @@ func (z *Zookeeper) Description() string {
 // Gather reads stats from all configured servers accumulates stats
 func (z *Zookeeper) Gather(acc telegraf.Accumulator) error {
 	if len(z.Servers) == 0 {
-		return nil
+		z.Servers = []string{":2181"}
 	}
 
 	for _, serverAddress := range z.Servers {