Allow forwarded_port 4243 to be overridden; Listen on 127.0.0.1 only by default.

The Docker port allows arbitrary remote code execution inside the VM, so
we don't want that turned on by default.

Unfortunately, this is still going to be a problem for users of the
vmware_fusion provider, since it apparently ignores the :host_ip
parameter:

    https://github.com/mitchellh/vagrant/issues/3916

Adding :id => "docker" allows downstream users of the box to disable the
port-forward using something like this in their Vagrantfile:

    config.vm.network "forwarded_port", guest: 4243, host: 4243, id: "docker", disabled: true