Fix ssh command concat error

the precedence concatenation operator (`.`) is higher than the ternary operator (`?:`), so the ssh command concat expresion has a fault.

an example:

```php
php > $host = 'foo';
php > $cmd = 'ssh ' . $host ?: 'bar';
php > echo $cmd, PHP_EOL;
ssh foo
php > $host = '';
php > $cmd = 'ssh ' . $host ?: 'bar';
php > echo $cmd, PHP_EOL;
ssh
php >
```