Fixes #4902: Use a new user, 'admin', in the default Rudder users file a...

...nd clean up the instructions

Ticket: http://www.rudder-project.org/redmine/issues/4902

To be merged in 2.11
I see no other reference to this specific text in another repo:
https://github.com/search?q=%22This+file+was+originally+made+from+Rudder+2.4+or+later%22&ref=searchresults&type=Code