We are not going to cover generic non-OpenStack service checks in depth here. There is plenty of information you can search for on the Internet that can guide you on generic service checks. We will put these and the OpenStack service checks into /etc/nagios/conf.d/nagios_service.cfg. For OpenStack, it is important to at least add a host load and a disk usage check for each host. OpenStack can consume an excessive amount of disk space and processor load, and the whole cluster can become cranky very quickly if either is used beyond one of the hosts' capacity. There are many other generic checks that can and maybe should be added to your OpenStack hosts, though you will have to research others and choose the checks that you deem advantageous. Here are examples of the configurations for checking the load and disk space on /var:
define service {
check_command check_nrpe!load5
host_name control
normal_check_interval 5
service_description 5 minute load average
use...