Expect ValidationError on an empty user update

Applying an empty change on user attributes should raise a
ValidationError.

This is a requirement for:
https://review.openstack.org/#/c/210004/

Depends-On: I5ab58fd64f4745a2e558392bff649b05401ed57a
Change-Id: I778bb408ba1f3b58037b77e56e33ee0020156c86
Closes-Bug: 1482279

trove/tests/api/users.py

@@ -24,6 +24,7 @@ from proboscis.asserts import fail
 from proboscis import before_class
 from proboscis import test
 from troveclient.compat import exceptions
+from troveclient.openstack.common.apiclient.exceptions import ValidationError
 
 from trove import tests
 from trove.tests.api.databases import TestDatabases
@@ -318,7 +319,7 @@ class TestUsers(object):
                       "host": hostname, "databases": []})
         self.dbaas.users.create(instance_info.id, users)
         user_new = {}
-        assert_raises(Exception,
+        assert_raises(ValidationError,
                       self.dbaas.users.update_attributes, instance_info.id,
                       username, user_new, hostname)
         # The last_http_code doesn't have to be checked, since the exception