Updated openstack/openstack
Project: openstack/oslo-incubator  725b14d2fb773c2f4629c54d53a211dd4e6fdab1

Use immutable arg rather mutable arg

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: Ie036e8203deb9ab10ffb7bce1db6cda6c3672a0c