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
Indeed, it was for a migration from 2.3 to 2.4. Since we don't support 2.3 anymore, it can safely be removed!