sshPort: configurable ssh port for distributed deployment

http://nodejs.netease.com/topic/5315bdf9104ca22b4237569d

Sometimes we change ssh port for security. This patch works for configurable ssh port in servers.json:

    {"id": "chat-server-1", "host": "192.168.1.2", "sshPort": 12121, "port": 4010, "frontend": false}