Implement additional and custom Nagios services support for hosts

[andrey-malets]

Along with traditional services like NRPE, pings, HTTP, SSH and other there are more host-specific checks and services for some hosts which must be implemented through custom checks and/or custom Nagios service definitions (e.g. HTTP on custom port, MP LAN status checks for services etc). We should develop a way to add those custom services easily and transparently to corresponding hosts in the config file.