Add support for modifying default host and service definitions

[Sandor Zeestraten]

The charm uses the default host/service definitions `generic-host` defined in `generic-host_nagios2.cfg` and `generic-service` defined in `generic-service_nagios2.cfg` for all hosts and services.

It lacks both the ability to:
1. Change the options in the default definitions `generic-host` and `generic-service`
2. Change the definition a host/service should inherit to something else than `generic-host` and `generic-service`.

The usage of `generic-host` and `generic-service` are currently hardcoded in `hooks/common.py`.

This together with #1864968 blocks the ability to enable notifications for schedule downtimes which requires the option `notification_options` to include `s` in host and service definitions.

See https://assets.nagios.com/downloads/nagioscore/docs/nagioscore/3/en/objectdefinitions.html for definitions.

[Eric Chen (eric-chen)]

This charm is no longer being actively maintained. Please consider using the new Canonical Observability Stack instead.
(https://charmhub.io/topics/canonical-observability-stack)
I will close this feature request