Allow to install the NGINX agent plugin

From the checkmk documentation:

Once this works, you need to install the agent plugin nginx_status into your agent's plugins directory (usually /usr/lib/check_mk_agent/plugins). The plugin tries to autodetect all running NGINX servers. If that fails for some reason you need to create the configuration file /etc/check_mk/nginx_status.cfg and configure your servers in that file. You find an example of that file in your agent directory (share/check_mk/agents).

This change allows us to optionally install the nginx_status plugin to get more in-depth statistics when available.

The files

• nginx_status.py
• nginx_status.cfg

are provided via checkmk.