Enable workers on debian systems

These commits solve the following problems and enable a successful install on Ubuntu (and probably debian) systems. Tested on Ubuntu Precise (12.04)

- no longer deletes the /etc/default/puppet-dashboard file on Debian os families as it is sourced by the puppet-dashboard-workers config
- enables the puppet-dashboard-workers service on Debian os families
- Provides a puppet-dashboard-workers config template 
- provides several variables for configuring the dashboard works service. Including
  - number of workers
  - enable/disable
  - dashboard_workers_service name 
  - dashboard workers config file path

Without this patch, no workers are started and the dashboard never collects reports.

*edit*

Oh, also added the spool directory to the list of directories chowned for the dashboard user...if this is not done, when the master submits reports it gets a 406 error.