Remove superfluous nagios/{files,map.jinja}

Files in nagios/files have not been used anywhere and have, in fact, been
duplicated in nagios/server/files and nagios/map.jinja is also not used.
Conditionally use distribution specific settings
Refactor states into smaller units

This patch splits the following into smaller units:

 * nagios.nrpe into .nrpe.server and .nrpe.plugin
 * nagios.server state into .server and .plugins

It also uses only a single map to keep all settings.
Configure NRPE with pillars
Do no install nrpe plugin Recommends on Debian