Smaller install of Nagios, that just monitors a single cluster of servers.

Allow for smaller installs of nagios, rather than monitoring an entire environment.  Some environments get huge.

Roles that have not been assigned to servers are filtered out of the hostgroups list, which cleans up the web UI a bit.  

Sorry for the double-pull-request.  I realized I left a relic of something else I was working on, and backed out.