Updated openstack/openstack
Project: openstack/ceilometer  4bfa2f4a7d1dc970e1cae25991d9af9a2fb98b81

Use None instead of mutables in method params defaults

Mutables in the method params defaults might cause errors and
that's why it's anti-pattern in most of the cases and should be
removed.

Change-Id: Ifb59a16c927a4bc285f7b70fac87f719054020f4