[Reviewer: Andy] Make the list of supported Cx vendor IDs configurable

Fixes #117 

Andy, please can you review my fix to make the list of supported Cx vendor IDs configurable.  It can now be overridden in `local_settings.py` to remove ETSI from the list.  I've tested both the default behavior and the overridden behavior against OpenIMS HSS and checked the flows are as expected and that OpenIMS HSS accepts them.