Bugfix: AttributeError when run tc055

Bugfix: AttributeError when run tc055

JIRA: YARDSTICK-662

When I run tc055, I got an error, see log:
Traceback (most recent call last):
      File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in
      _bootstrap
          self.run()
        File "/usr/lib/python2.7/multiprocessing/process.py", line 114,
        in run
          self._target(*self._args, **self._kwargs)
        File
      "/usr/local/lib/python2.7/dist-packages/yardstick/benchmark/runners/iteration.py",
      line 46, in _worker_process
          initial_rate = options_cfg.get("rate", 100)
      AttributeError: 'NoneType' object has no attribute 'get'

This is because in the former patch, we get 'options' by scenario_cfg['options'], it is
unsafe since some test case do not have 'options' field. For tc055, 'options' is None.

Change-Id: I18a4a7954c18c609f422da403fe65c4739c93648
Signed-off-by: chenjiankun <chenjiankun1@huawei.com>