Basic E2E testing works well now

Hi, @asonas 

This PR enables e2e testing.

Just install `boot2docker` in your Mac and run `rake spec`. Commands and outputs will be shown by `DEBUG=1 rake spec`.

Note: I want to do the CI, so please re-register this repo `https://github.com/asonas/capistrano3-puppet` to wercker build...
(This is because only @asonas has the webhook privileges for the repo, not for me, just a contributor.)

Thank you!