Add ability to port-forward TCP docker service to host machine

Vagrantfile will check if $expose_docker_tcp is set in config.rb, and
will forwad the docker port (tcp/4243) to the specified port on the host.

Service for docker to listen on port 4243 added to user-data.
Merge pull request #100 from mattbailey/master

Add ability to port-forward TCP docker service to host machine