etcd-tester: fix, clean up multiple things (#5462)

* etcd-tester: more logging, fix typo

* etcd-tester: fix prevCompactRev scope

Fix https://github.com/coreos/etcd/issues/5440.

* etcd-tester: move utils to bottom, clean up logs

And remove stresser operation inside defrag

* etcd-tester: separate update revision call

* etcd-tester: fix cleanup when case is -1

tools/functional-tester/etcd-tester/cluster.go

@@ -175,6 +175,7 @@ func (c *cluster) WaitHealth() error {
 		if err == nil {
 			return nil
 		}
+		plog.Warningf("#%d setHealthKey error (%v)", i, err)
 		time.Sleep(time.Second)
 	}
 	return err
@@ -271,7 +272,7 @@ func setHealthKey(us []string) error {
 		cancel()
 		conn.Close()
 		if err != nil {
-			return err
+			return fmt.Errorf("%v (%s)", err, u)
 		}
 	}
 	return nil