Fixing decorators for rally.benchmark.base.Scenario

This patch fixes a couple of decorators for the base scenario class from
@staticmethod to @classmethod so that one can use the openstack clients
defined in class_init() from those decorated methods.

Closes-Bug: #1234961
Blueprint test-engine-remove-pytest

Change-Id: I239c2df06412f840aab325be3529c04ade13529b
Merge "Fixing decorators for rally.benchmark.base.Scenario"