Added ability to exclude hosts on the chef server from being monitored, fixed _default hostgroup issue when using multi_environment_mode 

The following changes allow you to define an array ['nagios']['server']['exclude_hosts'] which will then be excluded from the search that pulls the nodes to be monitored. Slightly crude, but it works and I've seen other people mention that it would be a nice feature to have.

I also found that if I was using multi_environment_mode then my Nagios config would break because there was no "_default" hostgroup defined (my Nagios server is in the _default environment). I initially worked around this by creating a _default search item in the nagios_hostgroups data bag, but instead have moved this into an if block in the hostgroups.cfg which will create it when in multi_environment_mode.