Separate Sahara scenario tests into sahara-tests repository

We've discussed on summit and after it that we need to have a tool to acceptance testing Sahara from different OpenStack releases. The scenario testing framework developed in the end of Kilo cycle fits this needs.

So, the plan is to extract scenario tests to the separated repo and wrap it to be able to use as a testing utility. It should eventually work well with different versions of OpenStack (last two releases + current master).