I copy-pasted wrong part of traceback in previous comment. Correct one is
Traceback (most recent call last):
  File "/opt/stack/keystone/keystone/tests/test_backend_ldap.py", line 1004, in test_delete_domain_with_user_added
    self.assignment_api.delete_domain(domain['id'])
  File "/opt/stack/keystone/keystone/assignment/core.py", line 284, in delete_domain
    self.driver.delete_domain(domain_id)
  File "/opt/stack/keystone/keystone/assignment/backends/sql.py", line 559, in delete_domain
    session.flush()
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/session.py", line 1718, in flush
    self._flush(objects)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/session.py", line 1789, in _flush
    flush_context.execute()
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/unitofwork.py", line 331, in execute
    rec.execute(self)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/unitofwork.py", line 498, in execute
    uow
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/persistence.py", line 115, in delete_obj
    cached_connections, mapper, table, delete)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/persistence.py", line 671, in _emit_delete_statements
    connection.execute(statement, del_objects)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1449, in execute
    params)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1584, in _execute_clauseelement
    compiled_sql, distilled_params
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1698, in _execute_context
    context)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1691, in _execute_context
    context)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/default.py", line 331, in do_execute
    cursor.execute(statement, parameters)
  File "/usr/lib/python2.7/dist-packages/MySQLdb/cursors.py", line 174, in execute
    self.errorhandler(self, exc, value)
  File "/usr/lib/python2.7/dist-packages/MySQLdb/connections.py", line 36, in defaulterrorhandler
    raise errorclass, errorvalue
IntegrityError: (IntegrityError) (1451, 'Cannot delete or update a parent row: a foreign key constraint fails (`keystone_test`.`project`, CONSTRAINT `project_ibfk_1` FOREIGN KEY (`domain_id`) REFERENCES `domain` (`id`))') 'DELETE FROM domain WHERE domain.id = %s' ('6ec418ea6e6b41f7ac6c6b916819e913',)
-----------------------------------------------------------------------------------------------------------------------------------------
When i used your test case the result was almost the same
Traceback (most recent call last):
  File "/opt/stack/keystone/keystone/tests/test_backend_ldap.py", line 1002, in test_delete_domain_with_user_added
    self.assignment_api.delete_domain(domain['id'])
  File "/opt/stack/keystone/keystone/assignment/core.py", line 284, in delete_domain
    self.driver.delete_domain(domain_id)
  File "/opt/stack/keystone/keystone/assignment/backends/sql.py", line 559, in delete_domain
    session.flush()
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/session.py", line 1718, in flush
    self._flush(objects)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/session.py", line 1789, in _flush
    flush_context.execute()
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/unitofwork.py", line 331, in execute
    rec.execute(self)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/unitofwork.py", line 498, in execute
    uow
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/persistence.py", line 115, in delete_obj
    cached_connections, mapper, table, delete)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/orm/persistence.py", line 671, in _emit_delete_statements
    connection.execute(statement, del_objects)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1449, in execute
    params)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1584, in _execute_clauseelement
    compiled_sql, distilled_params
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1698, in _execute_context
    context)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/base.py", line 1691, in _execute_context
    context)
  File "/usr/local/lib/python2.7/dist-packages/SQLAlchemy-0.7.9-py2.7-linux-x86_64.egg/sqlalchemy/engine/default.py", line 331, in do_execute
    cursor.execute(statement, parameters)
  File "/usr/lib/python2.7/dist-packages/MySQLdb/cursors.py", line 174, in execute
    self.errorhandler(self, exc, value)
  File "/usr/lib/python2.7/dist-packages/MySQLdb/connections.py", line 36, in defaulterrorhandler
    raise errorclass, errorvalue
IntegrityError: (IntegrityError) (1451, 'Cannot delete or update a parent row: a foreign key constraint fails (`keystone_test`.`user_domain_metadata`, CONSTRAINT `user_domain_metadata_ibfk_1` FOREIGN KEY (`domain_id`) REFERENCES `domain` (`id`))') 'DELETE FROM domain WHERE domain.id = %s' ('6f6588ef7a154b628de9c8458652af66',)