Add test cases for default_config_files
Add test cases for default_config_files

It's pretty important we actually test the behaviour of falling back to
a default list of config files. Very surprising that we weren't alrady
testing this.

Change-Id: I903384247480946cc6aed38528695a4b89e717d2

citrix-openstack-build/oslo.config,shad7/oslo.config,varunarya10/oslo.config,metacloud/oslo.config,citrix-openstack/build-oslo.config,tianshan/oslo.config,magic0704/oslo.config,JioCloud/oslo.config