[refactor] common/test_config: Remove duplicated config declaration in tests
[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/collectd-ceilometer-plugin,openstack/collectd-ceilometer-plugin