Fix account generator error message

The user dict does not contain a "user" field.

TrivialFix

Change-Id: Ie4afa7947d750c85e5c7a04b5ef7155086aacace
diff --git a/tempest/cmd/account_generator.py b/tempest/cmd/account_generator.py
index a154d0b..5fab961 100755
--- a/tempest/cmd/account_generator.py
+++ b/tempest/cmd/account_generator.py
@@ -262,7 +262,7 @@
             user = identity.get_user_by_username(tenants_admin,
                                                  tenant['id'], u['name'])
         except tempest.lib.exceptions.NotFound:
-            LOG.error("User: %s - not found" % u['user'])
+            LOG.error("User: %s - not found" % u['name'])
             continue
         for r in u['role_ids']:
             try: