Remove base test case from test_config.
Remove base test case from test_config.

Remove collectd_ceilometer.tests.base.TestCase from base classes
of collectd_ceilometer.tests.base.test_config test cases.

Related-Bug: #1615349
Change-Id: Ic407752a07fb96a5fc431e4253e976148800b5ea

openstack/collectd-ceilometer-plugin,openstack/collectd-ceilometer-plugin