add_is_default_to_subnetpool migration not postgresql-compliant

add_is_default_to_subnetpool migration[1] is not working on PostgreSQL with a non empty table because it adds a new column non-nullable with no default value and PostgreSQL cannot deduce it.

It doesn't affect MySQL which uses False as default value (wtf)

[1] neutron/db/migration/alembic_migrations/versions/mitaka/expand/13cfb89f881a_add_is_default_to_subnetpool.py
[2] stacktrace

user@host:/opt/stack/neutron$ neutron-db-manage --config-file ~/a upgrade head
No handlers could be found for logger "oslo_config.cfg"
INFO [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO [alembic.runtime.migration] Will assume transactional DDL.
  Running upgrade for neutron ...
INFO [alembic.runtime.migration] Context impl PostgresqlImpl.
INFO [alembic.runtime.migration] Will assume transactional DDL.
INFO [alembic.runtime.migration] Running upgrade 34af2b5c5a59 -> 59cb5b6cf4d, Add availability zone
INFO [alembic.runtime.migration] Running upgrade 59cb5b6cf4d -> 13cfb89f881a, add is_default to subnetpool
Traceback (most recent call last):
  File "/opt/stack/neutron/.tox/py27/bin/neutron-db-manage", line 10, in <module>
    sys.exit(main())
  File "/opt/stack/neutron/neutron/db/migration/cli.py", line 749, in main
    return_val |= bool(CONF.command.func(config, CONF.command.name))
  File "/opt/stack/neutron/neutron/db/migration/cli.py", line 225, in do_upgrade
    desc=branch, sql=CONF.command.sql)
  File "/opt/stack/neutron/neutron/db/migration/cli.py", line 127, in do_alembic_command
    getattr(alembic_command, cmd)(config, *args, **kwargs)
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/alembic/command.py", line 174, in upgrade
    script.run_env()
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/alembic/script/base.py", line 397, in run_env
    util.load_python_file(self.dir, 'env.py')
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/alembic/util/pyfiles.py", line 81, in load_python_file
    module = load_module_py(module_id, path)
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/alembic/util/compat.py", line 79, in load_module_py
    mod = imp.load_source(module_id, path, fp)
  File "/opt/stack/neutron/neutron/db/migration/alembic_migrations/env.py", line 126, in <module>
    run_migrations_online()
  File "/opt/stack/neutron/neutron/db/migration/alembic_migrations/env.py", line 120, in run_migrations_online
    context.run_migrations()
  File "<string>", line 8, in run_migrations
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/alembic/runtime/environment.py", line 797, in run_migrations
    self.get_context().run_migrations(**kw)
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/alembic/runtime/migration.py", line 312, in run_migrations
    step.migration_fn(**kw)
  File "/opt/stack/neutron/neutron/db/migration/alembic_migrations/versions/mitaka/expand/13cfb89f881a_add_is_default_to_subnetpool.py", line 36, in upgrade
    nullable=False))
  File "<string>", line 8, in add_column
  File "<string>", line 3, in add_column
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/alembic/operations/ops.py", line 1535, in add_column
    return operations.invoke(op)
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/alembic/operations/base.py", line 318, in invoke
    return fn(self, operation)
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/alembic/operations/toimpl.py", line 123, in add_column
    schema=schema
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/alembic/ddl/impl.py", line 172, in add_column
    self._exec(base.AddColumn(table_name, column, schema=schema))
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/alembic/ddl/impl.py", line 118, in _exec
    return conn.execute(construct, *multiparams, **params)
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 914, in execute
    return meth(self, multiparams, params)
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/sql/ddl.py", line 68, in _execute_on_connection
    return connection._execute_ddl(self, multiparams, params)
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 968, in _execute_ddl
    compiled
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1146, in _execute_context
    context)
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1337, in _handle_dbapi_exception
    util.raise_from_cause(newraise, exc_info)
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 200, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1139, in _execute_context
    context)
  File "/opt/stack/neutron/.tox/py27/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 450, in do_execute
    cursor.execute(statement, parameters)
oslo_db.exception.DBError: (psycopg2.IntegrityError) column "is_default" contains null values
 [SQL: 'ALTER TABLE subnetpools ADD COLUMN is_default BOOLEAN NOT NULL']