Switch tests to use commercetools demo accounts

Problem/Motivation

In the current implementation, the module uses a private commercetools project to run functional tests. So all developers who want to contribute on extending the tests, need access (client_id + client_secret) to this private commercetools project, to be able to regenerate the mocked API responses (tests asset files).

Proposed resolution

Now we have two demo accounts, provided by commercetools, that are publicly accessible. So, would be great to switch functional tests to use one of the public accounts, to give any developer the ability to work with extending the tests while implementing new features, without asking for the credentials from the private commercetools project.