init() method for benchmark scenario classes

One of the core system parts which is still partly implemented is the support of the init() methods defined in benchmark scenario classes, which should be launched by the scenario runner before the actual benchmark scenarios. init()'s can be used to allocate some resources needed for benchmarking, e.g. prepared servers or floating IPs.

2 main problems to fix in scope of this blueprint are the following:
1) Passing the context (which stores the links to resources created in init()) to a benchmark scenario which is run in a multiprocessing pool;
2) Creating the resources in such a way that they can be further used from benchmark scenarios (the problem is that different temporary users are used in different benchmark scenario runs)