title: Packer loops through ports when attempting to establish SSH connection
body: <p>When Packer reaches the  "Waiting for SSH to become available..." step.</p>
  
  <p>My logs show</p>
  
  <pre><code>14:07:29 [INFO] Attempting SSH connection...
  14:07:29 reconnecting to TCP connection for SSH
  14:07:29 handshaking with SSH
  14:07:29 handshake error: ssh: handshake failed: read tcp 127.0.0.1:60372-&gt;127.0.0.1:3057: read: connection reset by peer
  14:07:29 [DEBUG] SSH handshake err: ssh: handshake failed: read tcp 127.0.0.1:60372-&gt;127.0.0.1:3057: read: connection reset by peer
  
  14:07:36 [INFO] Attempting SSH connection...
  14:07:36 reconnecting to TCP connection for SSH
  14:07:36 handshaking with SSH
  14:07:36 handshake error: ssh: handshake failed: read tcp 127.0.0.1:60376-&gt;127.0.0.1:3057: read: connection reset by peer
  14:07:36 [DEBUG] SSH handshake err: ssh: handshake failed: read tcp 127.0.0.1:60376-&gt;127.0.0.1:3057: read: connection reset by peer
  </code></pre>
  
  <p>Note a different port on each attempt. </p>
  
  <pre><code>60372
  60376
  </code></pre>
  
  <p>Packer is trying a new port, every 7 seconds.</p>
  
  <p>Is there a way to configure the ports before or during the build to avoid this try/fail approach?</p>