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 agree that these details are quite advanced for this file, but I think it is important to mention here that this level of authorization is available. Maybe just add a one-liner mentoining it?