Updated openstack/openstack
Project: openstack/sahara  b463c64bdfd738b0b7d4a18e19cebd4bf33608f5

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: If3a10d58e6cd792a2011c177c49d3b865a7421ff