Add independent check plugin defined type

This defined type is independent of NRPE to make it less confusing when using other client that reference the check plugins (scripts) or check plugins (scripts) that are not used via NRPE but executed locally from the Icinga 2 Core node.

Created a new parameter $checkplugin_libdir   = '/usr/lib/nagios/plugins' to keep backwards compatibility with the nrpe::plugins manifest.