Add missing translation import to common.auth.py

This utility was refactored, but the refactor didn't include a
required import statement:

  Ie7e156a83006c1578c87d862cb4d1d948a800809

This wasn't caught because non of our unit tests were trying to
build authentication payloads with a bogus scope.

Change-Id: I43f72d12f3eb57af69c4b40258baa3b11d0a6a54

keystone/tests/common/auth.py

@@ -10,6 +10,8 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+from keystone.i18n import _
+
 
 class AuthTestMixin(object):
     """To hold auth building helper functions."""