qa/tasks/vstart_runner: do not teardown test_path if "create-cluster-only"

qa/tasks/vstart_runner: do not teardown test_path if "create-cluster-only"

otherwise we could be removing a "None" directory when tearing down the cluster,
and have following failure:

Exception ignored in: <bound method LocalContext.__del__ of <__main__.LocalContext object at 0x7f99fd4a6cc0>>
Traceback (most recent call last):
  File "../qa/tasks/vstart_runner.py", line 1189, in __del__
    shutil.rmtree(self.teuthology_config['test_path'])
  File "/tmp/tmp.mmM2ugspuR/venv/lib/python3.6/shutil.py", line 477, in rmtree
    onerror(os.lstat, path, sys.exc_info())
  File "/tmp/tmp.mmM2ugspuR/venv/lib/python3.6/shutil.py", line 475, in rmtree
    orig_st = os.lstat(path)
TypeError: lstat: path should be string, bytes or os.PathLike, not NoneType

Signed-off-by: Kefu Chai <kchai@redhat.com>