Added a parameter for picking whether to install the Nagios plugins packages on nodes with the icinga2::server class applied.
Wrapped the server plugin install in an if statement so that installing the plugins packages depends on the parameter being set to true.
Added notes in the README about how to stop the icinga2::server class from installing the plugins packages.
Merge branch 'fix/duplicate_plugin_package_resources' into develop
Merge branch 'develop' into refactor/rename_client_to_nrpe

Merge the develop branch in so we can get the parameter that allows us to not install the plugin packages via the ::server class.
Renamed the icinga2::client class to icinga2::nrpe; this is mainly to accomodate other potential tyes of clients in the future, like NSCA or the Icinga 2 agent.
Deleted the duplicate client.pp and client/ directories.
Merge branch 'refactor/rename_client_to_nrpe' into develop