Add benchmark scenarios base class

Every benchmark should be subclass of base.Scenario.
This allows us to implement next things:

*) Auto discovering of benchmark Scenarios
*) Init & CleanUp functionality for benchmark Scenarios
*) All becnhmark Scenarios have built-in os python clients

blueprint test-engine-remove-pytest

Change-Id: I76f70b5bf3d35c3db8bb4d6f94f179c91f6f36a9