Protected matcher import

In python the names starts with '_'should not be used from another
package/distribution without a good reason.

The matchers are reachable from a not protected namespace.

Change-Id: I8892599edcbe7c4dd1cebea2c99e81a3221f8ef5
diff --git a/tempest/api/identity/admin/test_users.py b/tempest/api/identity/admin/test_users.py
index 6f90b04..cbbe05d 100644
--- a/tempest/api/identity/admin/test_users.py
+++ b/tempest/api/identity/admin/test_users.py
@@ -15,7 +15,7 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-from testtools.matchers._basic import Contains
+from testtools.matchers import Contains
 
 from tempest.api.identity import base
 from tempest.common.utils.data_utils import rand_name