Refactor set up code in tests to setUp methods

[Emma Foley]

* Task involves finding the common set-up code in unit tests and moving it to the setUp() functions.
* This helps keep the test code clean and and easy to grok

Examples of refactoring required:

* All tests in ``tests/common/test_config.py`` have ``config = settings.Config._decorated()``, this can be moved to the setUp() function and set as ``self.config = settings.Config._decorated()``, then accessed as ``self.config`` in all tests.
* ``tests/common/test_logger.py``:
  ** handler = CollectdLogHandler(collectd=collectd)
     logger = logging.Logger('some_logger')
     logger.addHandler(handler)
  ** Refactor to ``self.logger`` in tests

  ** etc

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/500819
Committed: https://git.openstack.org/cgit/openstack/collectd-ceilometer-plugin/commit/?id=e4c646942957595a7ff12dff273582cf0b77f542
Submitter: Jenkins
Branch: master

commit e4c646942957595a7ff12dff273582cf0b77f542
Author: Emma Foley <email address hidden>
Date: Tue Sep 5 13:09:25 2017 +0000

    [refactor] common/test_config: Remove duplicated config declaration in tests

    * Added setUp() call
    * Moved declaration of config to setUp()
    * config -> self.config

    Change-Id: I07a663552c5b6333630f381961411acd8ec39f73
    Partial-Bug: #1714012

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/501361
Committed: https://git.openstack.org/cgit/openstack/collectd-ceilometer-plugin/commit/?id=f6fd19b620c3ae0d252fa99c1b3f063ff6851164
Submitter: Jenkins
Branch: master

commit f6fd19b620c3ae0d252fa99c1b3f063ff6851164
Author: Emma Foley <email address hidden>
Date: Wed Sep 6 17:10:36 2017 +0000

    [refactor] common/test_logger.py: move common code to setUp()

    * Add setUp()
    ** Add self.handler, self.collectd, self.config, self.logger members
    * remove __init__()

    Change-Id: Ic8e52f34fc4a479f34bf714bba93008a3c8f3d25
    Partial-Bug: #1714012