Change the playground info for connecting to TiDB (pingcap#1133)

A `tiup playground nightly` results in:
```
To connect TiDB: mysql --host 127.0.0.1 --port 4000 -u root
```

Running this doesn't work:
```
$ mysql --host 127.0.0.1 --port 4000 -u root
ERROR 1045 (28000): Access denied for user 'root'@'127.0.0.1' (using password: YES)
```

This happens if a `~/.mylogin.cnf` is present or if a password is configured in `~/.my.cnf`.

```
$ mysql_config_editor print
[client]
password = *****

```

Adding a `-p` to `mysql` will cause this to prompt for passwords, just pressing
enter then results in a working connection.

Another option would be to set a password for the root user and use `-p<passwd>`
on the command.

components/playground/playground.go

@@ -872,7 +872,7 @@ func (p *Playground) bootCluster(ctx context.Context, env *environment.Environme
 		fmt.Println(color.GreenString("CLUSTER START SUCCESSFULLY, Enjoy it ^-^"))
 		for _, dbAddr := range succ {
 			ss := strings.Split(dbAddr, ":")
-			fmt.Println(color.GreenString("To connect TiDB: mysql --host %s --port %s -u root", ss[0], ss[1]))
+			fmt.Println(color.GreenString("To connect TiDB: mysql --host %s --port %s -u root -p (no password)", ss[0], ss[1]))
 		}
 	}