[WIP] Docker provisioner

So I started working on porting the code from [vocker](https://github.com/fgrehm/vocker) and although I haven't tested this yet I wanted to kick off the discussion about some aspects about it as the code from the plugin probably won't be merged on its current state.

### `vagrant docker` [command](https://github.com/fgrehm/vocker#vagrant-docker-command)

Vocker provides a convenience command for interacting with the Docker installation running within the VM but I'm not sure it makes sense to have that as part of Vagrant core. Besides that, I personally haven't been using it as much as I thought I would.
If you guys think it makes sense I can bring it back in

### vagrant-lxc specific code within the provider

I personally like to have isolated Docker instances inside other VMs and I found that "plain old lxc "gives me enough abstraction for that and it makes up for a really awesome alternative to VBox on Linux. 

I found my way to make it work from within vagrant-lxc containers but I had to add a conditional on the provisioner code and I'm not sure if Vagrant's core should have provider specific logic within it. I'm happy to drop that as long as we have a way of specifying from the provider that it should be skipped in case the provisioner is used.

### Running containers from the Vagrantfile

I'd love some feedback about the code that is already in place. It makes some assumptions about where to place container id files and it also creates the directories used as Docker volumes. While the automatic folder creation is a nice convenience, and I'm not 100% sure if that should be a responsibility of the plugin.

### Provisioner specs

I have some tests written on the plugin but I noticed that there is no automated tests for other provisioners and I suppose that there is a reason for that. I'm happy to bring them back if you guys think it is worth

### Docs

I haven't written any yet. Should we have that for 1.4 or should we wait a bit more as things are subject to change?

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There's probably more stuff you guys will find on the code so please take your time to review :)
I plan to do some testing on it later on tonight or tomorrow and I'm up for doing whatever it is needed to get this into 1.4 :D