Always init cfg.CONF before running a test

In all tests cfg.CONF is not (re)initialised correctly
By chance in ceilometer , the only method that fail when conf is not
initialised is cfg.CONF.find_file.
And because some tests do special initialisation (ie: the ones that test
ceilometer.service.prepare_service stuff), the method cfg.CONF.find_file
works when we run all tests.

But if you decide to run only one test (like below) that use find_file it fail
because cfg.CONF is not initialised.

  tox -epy27,pep8 api.v2

Not all configurations opts are impacted but only the CLI opts ones that are
set only when cfg.CONF() is called.

This change initialise cfg.CONF on each test (but the test can
continue to reinitialise it if needed)

Change-Id: I37842e945a7e2908e17d5ce886e17047a36818f7