Updated openstack/openstack
Project: openstack/oslo.db  d7da57c87838fcb6449ed85d0989d139a596d6fa

Avoid usage of mutables as default args

Passing mutable objects as default args is a known Python pitfall.
We'd better avoid this. This commit changes mutable default args with
None, then use 'arg = arg or []'.

Change-Id: Id91b53e03c17a19e94830b43fe0c4970c824ff70