Fix ScenarioRunnerTestCase:test_run_scenario_timeout
Fix ScenarioRunnerTestCase:test_run_scenario_timeout

This could mocks in wrong way multiprocessing wich may produce
non deterministic errors in gates.

Closes-Bug: #1286401
Change-Id: I66b98541777f712e1892a33734ffbbc0b6890f7e

amit0701/rally,openstack/rally,redhat-openstack/rally,cernops/rally,eayunstack/rally,gluke77/rally,pandeyop/rally,aforalee/RRally,shdowofdeath/rally,paboldin/rally,vponomaryov/rally,amit0701/rally,eayunstack/rally,afaheem88/rally,shdowofdeath/rally,varunarya10/rally,eonpatapon/rally,pandeyop/rally,yeming233/rally,vponomaryov/rally,vganapath/rally,vganapath/rally,paboldin/rally,amit0701/rally,yeming233/rally,vganapath/rally,gluke77/rally,vganapath/rally,openstack/rally,vefimova/rally,group-policy/rally,ytsarev/rally,group-policy/rally,openstack/rally,eayunstack/rally,aforalee/RRally,gluke77/rally,paboldin/rally,cernops/rally,aplanas/rally,afaheem88/rally,go-bears/rally,pyKun/rally,gluke77/rally,varunarya10/rally,redhat-openstack/rally,openstack/rally,aplanas/rally,pyKun/rally,go-bears/rally,ytsarev/rally,group-policy/rally,eonpatapon/rally,vefimova/rally