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/services/identity/json/token_client.py b/tempest/services/identity/json/token_client.py
index 93936bc..1e8b31e 100644
--- a/tempest/services/identity/json/token_client.py
+++ b/tempest/services/identity/json/token_client.py
@@ -13,6 +13,7 @@
 #    under the License.
 
 import json
+from tempest_lib import exceptions as lib_exc
 
 from tempest.common import service_client
 from tempest import config
@@ -87,7 +88,7 @@
 
         if resp.status in [401, 403]:
             resp_body = json.loads(resp_body)
-            raise exceptions.Unauthorized(resp_body['error']['message'])
+            raise lib_exc.Unauthorized(resp_body['error']['message'])
         elif resp.status not in [200, 201]:
             raise exceptions.IdentityError(
                 'Unexpected status code {0}'.format(resp.status))