ofswitch: appease pep8
fix conflict of variable name

careless bug of my previous patch.
ofswitch: fix unittest failure

fixes the following error:

----------------------------------------------------------------------
Traceback (most recent call last):
  File "neutron/tests/unit/ofswitch/test_ofs_neutron_agent.py", line
190, in test_setup_ofp_default_par
    self.ovs.setup_ofp()
  File "neutron/plugins/ofswitch/agent/ofs_neutron_agent.py", line 158,
in setup_ofp
    host = cfg.CONF.ofp_listen_host
  File
"/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/oslo/config/cfg.py",
line 1648, in __getattr__
    raise NoSuchOptError(name)
NoSuchOptError: no such option: ofp_listen_host
----------------------------------------------------------------------
Traceback (most recent call last):
  File "neutron/tests/unit/ofswitch/test_ofs_neutron_agent.py", line
225, in test_setup_ofp_with_except
    self.ovs.setup_ofp()
  File "neutron/plugins/ofswitch/agent/ofs_neutron_agent.py", line 158,
in setup_ofp
    host = cfg.CONF.ofp_listen_host
  File
"/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/oslo/config/cfg.py",
line 1648, in __getattr__
    raise NoSuchOptError(name)
NoSuchOptError: no such option: ofp_listen_host