Update README.rst

Fixing broken link
Update README.rst
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
Drop suboptimal nagios server configuration
Drop _pkg suffix from nagios_server_pkg
Add nsca.client state

nsca allows the user to send service check results to a central monitoring
server running Nagios in a reasonably secure manner. The nsca.client state
installs and configured the client part of nsca to allow hosts to report
passive service checks.
Adjust second-level header underlines
Manage nagios server configuration