Disable repositories with rejected GPG keys during installation

* Fixes bnc#723019 and bnc#886572
* The included test case is pretty basic (not to say dumb or useless) but I did A LOT of monkey testing to verify that works as expected.
I used variables instead of ```let``` as a way to emphatize that the variables were in fact irrelevant and having no meaning or influence in the different test (although it probably only worked in my mind) :-)