Use cls instances instead of object in benchmarks

Using @classmethod as a base for scenarios was actually caused
by historical reasons (of using pytest as a base for benchmarks)
As we don't use it anymore (for a long time) we are able to simplify
benchmark framework and use cls instances and plain methods.

This is only the first step. That just make required changes and fix
unit tests. In next patchset I will refactor & clean exisitng unit tests.

blueprint refactor-scenarios-to-use-class-instances

Change-Id: Idd270635f70731ccf575174f8f96dc1e95cc0f8a