Updated Vagrantfile's box_url and added private IP.

In addition: moved 'lxc' related provision settings inside the lxc
provider block and slightly cleaned up code.
Merge branch 'refactor-vagrantfile2' of https://github.com/mikexstudios/chef-dokku into refactor
Clean up vagrantfile
Remove vagrant-lxc specific code from Vagrantfile

In order to make it work it'll require a bit more effort that I'm not
willing to handle right now