Set DASHBOARD_USER for worker processes

The current master branch fails on a permission error if you run dashboard as a non-default user. This happens because `DASHBOARD_USER` is specified in the dashboard configuration file, but not in the worker configuration file. I added this option to `workers.config.Debian.erb` in this pull request.

I also added missing `@` signs to variables in this file to fix two deprecation warnings in Puppet 3.