Change tempest Unauthorized exc to tempest-lib exc

This commit changes tempest.exceptions.Unauthorized to
tempest_lib.exceptions.Unauthorized. This is one of the migrating rest
client to tempest-lib works.

Change-Id: I7675700b53ddedc2deee024c6e1ab3c88e0e3683
diff --git a/tempest/api/compute/admin/test_services_negative.py b/tempest/api/compute/admin/test_services_negative.py
index bb19a39..39d0ee1 100644
--- a/tempest/api/compute/admin/test_services_negative.py
+++ b/tempest/api/compute/admin/test_services_negative.py
@@ -12,8 +12,9 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from tempest_lib import exceptions as lib_exc
+
 from tempest.api.compute import base
-from tempest import exceptions
 from tempest import test
 
 
@@ -31,7 +32,7 @@
 
     @test.attr(type=['negative', 'gate'])
     def test_list_services_with_non_admin_user(self):
-        self.assertRaises(exceptions.Unauthorized,
+        self.assertRaises(lib_exc.Unauthorized,
                           self.non_admin_client.list_services)
 
     @test.attr(type=['negative', 'gate'])