Adding a new configuration option :force_ssh_port (boolean)

This provides the following:
1- veewee won't create a nat rule for ssh in virtualbox, meaning that you're not forced to use nat for the interface. It allows things like :virtualbox => { :vm_options => [ 'nic1' => 'intnet', ]} in the definition.rb,
2- veewee won't try to guess the port for ssh but will take whatever value was giving in :ssh_host_port. That assumes that the port will work (if you're using intnet you'll have to redirect the port yourself)