Ok I didn't got that you wanted to change it only for the nagios server, sorry.
Still 2 concerns:
- Travis checks are all failing on one specific versions matrix  (https://travis-ci.org/example42/puppet-nagios/jobs/7678240) would you give them a look?
- I'm not sure you have predictable behaviour on the Nagios server: if the (common) nagios::target class is evaluated before the nagios one, you will have a nagios::host without the parents configuration.