Added node['munin']['server_list']

This allows finer grain control of the server list.  Including allowing for
the server to be outside this chef-server's control.

Use cases:

* Use chef-solo/zero to deploy testing VMs but wanting to report to
 different server(s).
* When migrating from another system (e.g. puppet) to Chef and the
 server(s) haven't been moved yet.