Change the default value of scenario_output

Change the default value of the scenario_output variable of the
_run_scenario_once function of the rally/benchmark/runners/base.py
module because there was no errors key in this variable when a scenario
failed with timeout.

Closes-Bug: 1336762
Change-Id: Ieebc2064bcfee3f2bd23ee40394f8117e9096f01