Use dotenv for configuration

Putting all configuration options into a .env file that is used by Rails, Capistrano, Vagrant and Chef. Also updated the Chef cookbook accordingly.

The provided .env.example can be used for a quick temporary install, e.g. Travis. `database.yml` and `settings.yml` now contain ENV variables and are under version control. 

We can also do the extra step do get rid of `settings.yml`, which is the cleaner solution.
Yes, a Rubocop recommendation. Use single quotes inside string interpolation.