Modify UT case for config in the common directory
Modify UT case for config in the common directory

add a test case for config.py in the tacker/config directory

Change-Id: Id789c959cf3ed40d8fa9b75e8d861ebf297eca4b

openstack/tacker,stackforge/tacker,openstack/tacker,openstack/tacker,stackforge/tacker