Fix unit test that break isolation

Unit test that require configuration should use the configuration
fixture and set_default so that configuration changes stay isolated
to the specific test.

Tests in test_base_test break this rule and cause race conditions
in other tests, fixing that.

Change-Id: I1c0b62cb29268fb4d34ab769eaf35139c4204509
diff --git a/tempest/tests/test_base_test.py b/tempest/tests/test_base_test.py
index 6c6f612..3ece11d 100644
--- a/tempest/tests/test_base_test.py
+++ b/tempest/tests/test_base_test.py
@@ -13,10 +13,10 @@
 #    under the License.
 
 import mock
+from oslo_config import cfg
 
 from tempest import clients
 from tempest.common import credentials_factory as credentials
-from tempest import config
 from tempest.lib.common import fixed_network
 from tempest import test
 from tempest.tests import base
@@ -28,8 +28,9 @@
         super(TestBaseTestCase, self).setUp()
         self.useFixture(fake_config.ConfigFixture())
         self.fixed_network_name = 'fixed-net'
-        config.CONF.compute.fixed_network_name = self.fixed_network_name
-        config.CONF.service_available.neutron = True
+        cfg.CONF.set_default('fixed_network_name', self.fixed_network_name,
+                             'compute')
+        cfg.CONF.set_default('neutron', True, 'service_available')
 
     @mock.patch.object(test.BaseTestCase, 'get_client_manager')
     @mock.patch.object(test.BaseTestCase, '_get_credentials_provider')
@@ -56,7 +57,7 @@
     def test_get_tenant_network_with_nova_net(self, mock_man, mock_iaa,
                                               mock_giv, mock_gtn, mock_gcp,
                                               mock_gcm):
-        config.CONF.service_available.neutron = False
+        cfg.CONF.set_default('neutron', False, 'service_available')
         mock_prov = mock.Mock()
         mock_admin_man = mock.Mock()
         mock_iaa.return_value = True