Sucessfully running Behat scenarios for OAuth sign-in

As mentioned in https://github.com/Sylius/Sylius/pull/354#issuecomment-26726162, the Behat scenarios for testing the OAuth sign-in are to complex/insecure so far. Thus I simply removed the critical ones. I kept only one scenario to test at least the reachability of the resource owner's login page from Sylius. You only need to specify an actual client ID + secret for Login with Amazon (configured with `http://localhost/login/check-amazon` as an allowed return URL) when installing via composer.

The use of Amazon serves as a PoC here. We should substitute it with another resource owner and include pre-configured credentials (client ID + secret) in `parameters.yml.dist`. Twitter configured with the Sylius Twitter account? Any other suggestions?