Don't mix VM templates and Chef cookbooks in the same folder

First of all: I find really good that the YAML VM templates are now stored in travis-cookbooks repository instead of travis-images (51cd07cb93f4bb67bd1eb19b9ef1f764cb0fd6f8). This will be of great help to keep things aligned on pull requests (but we'll also have to be careful to not forget to update the cookbook default attributes now).

But for sake of clarity, I do think it is not that good to store these templates aside all the cookbooks. I propose a simple `vm_templates` folder, but if same YAML templates are also used for the `host_worker` provisioning, why not something *more formally organized* like:

```
├── ci_environment
├── roles
├── spec
├── templates
│   ├── ci_environment
│   │   ├── worker.android.yml
│   │   ├── worker.erlang.yml
│   │   ├── worker.go.yml
│   │   ├── worker.haskell.yml
│   │   ├── worker.jvm.yml
│   │   ├── worker.node-js.yml
│   │   ├── worker.perl.yml
│   │   ├── worker.php.yml
│   │   ├── worker.python.yml
│   │   ├── worker.ruby.yml
│   │   └── worker.standard.yml
│   └── worker_host
│   │   ├── worker_host.yml
├── worker_host
```
Note: we also should consider switching to Chef roles one day.