Fix error handling on instance create

exception.BadRequest expects string as a 'message' argument, so we can't
pass ValueError without casting it to string.

Change-Id: Iff43c38dca596e694cc7c6bc20e71669d83e31e2

trove/instance/service.py

@@ -431,7 +431,7 @@ class InstanceController(wsgi.Controller):
             users = populate_users(body['instance'].get('users', []),
                                    database_names)
         except ValueError as ve:
-            raise exception.BadRequest(message=ve)
+            raise exception.BadRequest(message=str(ve))
         if slave_of_id and (databases or users):
             raise exception.ReplicaCreateWithUsersDatabasesError()