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
tests: initialize admin context after super().setUp call

New policy code requires that CONF is already initialized when policy
checks are invoked. It means that no Neutron context objects should be
created before BaseTestCase.setUp() is called that will set
configuration, among other things.

Ideally, we would just make sure that all test cases invoke
super().setUp() as the very first line of any subclass setUp() methods.
But since some of test cases require prior setup (like mocking extension
manager) before proceeding to base class, we end up with magically
shuffling context instantiations to occur below super() calls.

Change-Id: Ib1039ea1ff9480ca57f9cb0c917469d0d2ad28b8
Closes-Bug: #1400301
Merge "ofa_test_base: Fix NoSuchOptError in UT"
Merge "tests: initialize admin context after super().setUp call"