Load tests from `recipe.yaml` file

[therazix]

[happz]

As discussed on Thursday afternoon, post-lunch session, we are going to take a slight detour first: move functionality that should be shared out of discover plugins - git cloning, policies, libraries, etc. (not distgit though, that one will be left for later). We would also modify the internal API, as we will expect a discover plugin to return more info than today. Eventually, we would not need to run discover plugins' go() method when recipe is given, as recipe would provide all needed info to "restore" the discover product.