ofa_test_base: Fix NoSuchOptError in UT

Ensure setting up cfg.CONF.
Otherwise, depending on the execution order of tests,
some tests in test_ofa_neutron_agent can fail with
the following exception:

    NoSuchOptError: no such option: config_dir

The problem has been exposed by the recently merged change
for policy module, commit d83731a96d3b47fab1295de1fb53cfe30576619c .

Closes-Bug: #1392569
Change-Id: I0d38df0510b95160c67ea7b76ed413138fa3d770
Merge "ofa_test_base: Fix NoSuchOptError in UT"