Create a set_config behat step

We talked about it and it may help MDL-48613 and reduce the whole suite duration; we have many manual (well manual.. selenium or goutte) steps to create activities and setting admin settings values, we can just have a set_config behat step that sets config using PHP rather than interacting with the website.

I remember we talked about it but I haven't found any issue to work on it. I've just remembered about it while reviewing MDL-48715 which is being affected by this.