[Reviewer: Andy] Add ELLIS_USER option to override the Ellis login

This is a slightly more limited version of the "make usernames a UUID" approach I bounced off you yesterday. This would allow us to give our CI live tests independent users on staging, so that when they run `destroy_leaked_numbers` they won't trample over each others' lines, or the lines of any live tests we happen to be running manually - but we won't create a new user on each run, so won't leak lines.

I've also updated the default email and username from "System Test"/"system.test" to make it clearer that these are users automatically created by clearwater-live-test

Tested by running against my deployment and then checking the Ellis users database..