Added hostgroups and servicegroups parameters

I left the default parameters as they were (for backwards compatibility)... but, to me, they should be empty, specially the mysql_health... what do you think? Should I send you a PR with this?

Besides, I also got quite confused: why do you use the lib/facter/nagios_hostgroups.rb? It seems to pick up some files in /etc/nagios/facter/hostgroup*.conf, but there is nowhere inside your module that populates this directory. Do you need to populate it by hand? Why is this custom fact there? For the servers or for the clients?

Thanks!