Fix TypeError when creating swarm error message
$ rally task report --json d294daed-fca4-4cbd-b937-1a181c97c01b
Traceback (most recent call last):
File \"/usr/lib/python3.6/site-packages/rally/task/runner.py\", line 69, in _run_scenario_once
getattr(scenario_inst, method_name)(**scenario_kwargs)
File \"/usr/lib/python3.6/site-packages/rally_openstack/scenarios/magnum/swarm_containers.py\", line 64, in run
self._create_stack(manifest)
File \"/usr/lib/python3.6/site-packages/rally/task/atomic.py\", line 91, in func_atomic_actions
f = func(self, *args, **kwargs)
File \"/usr/lib/python3.6/site-packages/rally_openstack/scenarios/magnum/utils.py\", line 480, in _create_stack
+ \", output of stack ps:\\n\" + result)
TypeError: must be str, not bytes