Add test kitchen and tests for nginx, mysql and application configs

This PR adds test-kitchen with a `.kitchen.yml` file and configure it to use a standard
Vagrant provider with an Ubuntu 14.04 box. You can execute a test-kitchen test using:

```
$ bin/kitchen test
```

This will download and create the Vagrant box, provision it, run the tests and remove the image.

The tests are located in `test/integration/full-stack/serverspec`.
This was because there are no minitest/unittest assertions for these server tests.